For the second time in four years, W.A. (Tony) Boyle, the former president of the United Mine Workers union, yesterday was convicted of murdering union rival Joseph (Jock) Yablonsky and Yablonsky's wife and daughter.

The jury deliberated for four hours and 50 minutes Friday night and yesterday morning before returning at 9:50 a.m. to announce that they found the 76-year-old defendant guilty of three counts of first-degree murder. The counts carry automatic life sentences.

Yablonsky, who challenged Boyle for the UMW presidency in 1969, his wife, Margaret, and daughter, Charlotte, were slain in their Clarksville, Pa., home in the early morning hours of Dec. 31, 1969.

As David Wood, the jury foreman, read the verdict aloud in a declaratory tone, a single loud clap resounded in the courtroom. It came from the prosecution side, where the Yablonsky family also sat.

Boyle's family, for the first time in the trial, was permitted to sit in the front row, within two feet of him, but they did not look at each other as the verdict was read.

Boyle, like most others in the courtroom who had expected the jury to deliberate much longer, interpreted their relatively short deliberations as a sign and apparently was steeled for a guilty verdict. When it was read, he simply stared ahead vacantly and his face sagged.

His daughter, Antoinette Engebregson, appeared to have braced herself and held back tears. Boyle's brother, Richard, showed no emotion. His wife, Ethel, was not in the courtroom because she has been hospitalized with pneumonia.

Across the aisle, where FBI agents, Pennyslvania state police and the Yablonsky family crowded into the first two rows, Kenneth Yablonsky, the elder son of the slain man, who discovered the bodies of his family on Jan. 5, 1970, took off his glasses and wiped his eyes. His half-brother, Joseph (Chip) Yablonsky Jr., showed no emotion.

When Judge Francis J. Catania dismissed the jurors and adjourned the trial, Boyle, surrounded by his family and appearing numb, said only, "I'm disappointed," and told reporters he could say nothing more until he spoke to A. Charles Peruto, his lawyer.

Peruto said he would appeal the verdict, and as reporters flocked to special prosecutor Richard A. Sprague, the Boyle family, Peruto and his associate, Burton Rose, left the courtroom.

Sprague, speaking in the same measured, sober tone he has used throughout both murder trials, said, "A jury is a little more astute, a little more able to see what the issues are," than to accept "this ham acting, this burlesque, this attempt to laugh this case out of court." The words were the ones Sprague used in his closing argument to the jury Friday, when he accused Peruto of using theatrics to distort the facts of the case.

Because of Peruto's professional track record (until yesterday he claimed to have lost only one first-degree murder case, and that was to prosecutor Sprague) and because of his aggressive cross-examination of prosecution witnesses, some observers believed he had a chance to persuade the jury as to "reasonable doubt" about Boyle's connection to the murder plot.

In the end, however, the sober, methodical style of Sprague won out. (Boyle won a new trial a year ago when the Pennsylvania Supreme Court set aside the 1974 guilty verdict on grounds of judicial error.)

After 14 days of voluminous testimony, the closing arguments Friday provided the jury with an overview of the issues they had to decide.

In a voice that fluctuated from a whisper to shrieks, Peruto told the jurors that he was "scared to death" that they would be "carried away" with the blood-and-gore aspects of the prosecution's testimony.

At one point, he attempted one last time to destroy the credibility of former UMW official Albert Pass, the chief prosecution witness, who with William Turnblazer, another union official, testified that on June 23, 1969, Boyle told them to kill Yablonsky. Pass, who testified in this trial for the first time, said the order was given before a union executive board meeting. Turnblazer said it occurred after the meeting.

Peruto said Pass was careful to make his testimony just a bit different from Turnblazer's because Pass thought to himself, "If I match too perfectly with Turnblazer no one's gonna believe it." So there's just enough difference for Mr. Sprague to say, 'If he was just backing up Turnblazer, their stories would have been identical.'"

Turning from the jury to face Sprague, Peruto bowed from the waist and sneered at the special prosecutor, "That sucker play went out with high-button shoes."

Judge Catania allowed Boyle to remain free until a bail hearing scheduled for next Friday.