A federal court jury found yesterday that The Washington Post libeled Mobil Oil Corp. President William P. Tavoulareas in a 1979 article about his business dealings and awarded him $2,050,000 in damages.

The six-member jury made the award, the largest ever against the newspaper, after deliberating 18 1/2 hours over three days. But the jurors found that The Post had not libeled Tavoulareas' son, Peter, who also was a plaintiff in the suit.

In a companion case, the jury found that the elder Tavoulareas' former son-in-law, Dr. Philip Piro, an eye surgeon who was an initial source for the Nov. 30, 1979, article, had slandered both Tavoulareases. The jury awarded the Mobil president $5,000 in the Piro case and his son $1,000.

"It was a vindication of my family and my son and a vindication for my company," the elated, 62-year-old Mobil president told reporters on the courthouse steps. "It's a victory for responsible journalism."

His son, 32, the principal owner of a London-based shipping management firm, described the verdict as a "complete vindication for my father, for both of us. That's what this was all about." He exuberantly told reporters he immediately planned to "have one hell of a blowout" to celebrate.

Irving Younger, one of The Post's attorneys, said the newspaper will ask U.S. District Judge Oliver Gasch, who presided at the 21-day trial, to set aside the judgment. Boisfeuillet Jones Jr., The Post's counsel, said that if Gasch does not set it aside, the newspaper will appeal the verdict to the U.S. Court of Appeals.

"The jury made a mistake," Younger said. "I'm surprised they found liability on anybody," adding that he thought it was a "bad day for the jury system."

The Tavoulareases claimed in their suit that the 80-inch-long Nov. 30 article, which detailed the creation of the younger Tavoulareas' firm, Atlas Maritime Co., defamed and embarrassed them and held them up to ridicule by suggesting that the elder Tavoulareas had used his Mobil influence to establish his son in business. They asked for $50 million in damages from The Post and $20 million from Piro.

By its verdict, the jury found that the newspaper, reporter Patrick Tyler who wrote the story, and special correspondent Sandy Golden, who led Tyler to an interview with Piro, knew the story was false when they published it or demonstrated a "reckless disregard" about the accuracy of the article.

The jury cleared two other Post defendants, executive editor Benjamin C. Bradlee and Bob Woodward, an assistant managing editor who did some initial reporting on the Tavoulareas story in 1976 and later had a hand in editing Tyler's story. The jurors also cleared The Post of the Tavoulareases' libel charges on a follow-up Dec. 1, 1979, story that Tyler wrote about allegations Rep. John D. Dingell (D-Mich.) made to the Securities and Exchange Commission about the Tavoulareases' business dealings.

Jury foreman Geoffrey T. Mott, a librarian at the Library of Congress, told a hushed courtroom that the jurors had awarded the elder Tavoulareas $250,000 in compensatory damages, which are intended to represent actual damages he incurred as a result of publication of the article. During the trial there was testimony that other publications wrote stories based on The Post article and that information from it was used in a Notre Dame law journal article and in a speech prepared by staff aides to Sen. Howard M. Metzenbaum (D-Ohio).

Mott also told Judge Gasch that the jury decided that punitive damages should be awarded against The Washington Post Co., which publishes the newspaper. According to the legal instructions Gasch gave the jury on Wednesday, punitive damages can be awarded "as a punishment for outrageous conduct."

Moments later, Gasch held a brief hearing to consider the amount of the punitive damages. The elder Tavoulareas testified that he and his son have already paid the New York-Washington law firm of Cadwalader, Wickersham & Taft $1.2 million in legal fees and expect that the total bill will be $2 million.

The Mobil president said that he has been paying 90 percent of the legal fees and his son 10 percent. The Tavoulareases' lead attorney, John J. Walsh of New York, also told the jury that The Post company had a net income last year of more than $32.7 million.

The jurors deliberated for another 10 minutes and then returned to announce they had awarded the Mobil president $1.8 million in punitive damages, exactly the amount of what he said his total legal fees will be.

The jurors quickly left the courthouse after delivering the verdict and declined to comment on their lengthy deliberations.

The elder Tavoulareas smiled faintly as the verdict was announced, but embraced his attorneys and later shook hands with Bradlee after the jury left the courtroom. Peter Tavoulareas raised his right fist as a sign of victory when the foreman read the verdict against Piro.

Tyler, 30, sat with his head bowed and his hands folded as the decision went against him. Later he said, "I'm obviously disappointed," but declined further comment, as did Bradlee and Woodward.

Piro's lawyer, David Machanic, said he had instructed his client not to talk to the press, "in view of the fact he's already gotten in trouble for speaking the truth."

The verdict is the latest decision that has gone against the American press in recent years, as judges and juries have been more inclined to question the processes by which publications and television news organizations piece together complex stories as well as the sources on which the stories are based.

Upon learning of the verdict, lawyers who specialize in defending the media in libel cases said they think juries are increasingly willing to punish the press for its conduct through large punitive damage awards, which could in the long run affect the media's willingness to investigate and report on complex stories.

"We could move to a new level of low-risk journalism," said Floyd Abrams, a New York lawyer who represents both The New York Times and the NBC television network.

However, Illinois lawyer Rex Carr, who last year won a major libel case against The Alton, Ill., Telegraph, said, "If it has a chilling effect on investigative reporting, that's what it's supposed to do . . . Investigate, yes, but don't publish unless it's true."

During the 16 days of testimony in Gasch's fourth-floor courtroom in the U.S. Courthouse, the jurors listened to 28 witnesses and to the testimony of 11 others that was read into the court record. In addition, about 160 documents--internal Mobil memoranda, tax records, newspaper clippings, reporters' notes, speeches and press releases--were introduced into evidence.

Although the drama centered on the accuracy of the two articles, it was in a broader sense a battle between two Fortune 500 companies. While Mobil was not a direct party to the Tavoulareases' suit, the giant oil company often had two Washington attorneys watching the proceedings as well as lawyers from its New York headquarters.

In the newspaper's view, the libel suit was a direct assault on its right to present a newsworthy story it believed it had accurately and thoroughly reported and edited. On the last day of testimony last Saturday, Post Executive Editor Bradlee sharply told Walsh, "You must understand what Mobil thinks is news is not necessarily what an editor of a newspaper thinks is news."

The Tavoulareases contended that the stories not only were false, but embarrassed them and held them up to ridicule. When the newspaper refused to retract the articles, the Tavoulareases responded by filing suit.