Richard D. Simmons, a vice chairman of Dun & Bradstreet Corp., yesterday was named president, chief operating officer and a director of the Washington Post Co., effective Sept. 1. Katharine Graham, who announced the appointment, will continue as board chairman and chief executive of the D.C.-based communications firm.

Simmons, 46, will fill a vacancy in the company's top management that has existed since Jan. 5 when former president Mark Meagher resigned the post he had held since November 1977. Meagher subsequently moved to Penthouse International publishing company as president.

To an interview yesterday, Simmons pointed to what he described as the "substantial similarities" of the Post Co. -- whose main businesses are newspaper and magazine publishing and television broadcasting -- and Dun & Bradstreet.

Based in New York, Dun & Bradstreet is best known for its corporate credit-rating operations. But D&B businesses also include six television stations, magazines such as Dun's Review, the Funk and Wagnalls publishing firm and Moody's Investors Service.

The D&B officer said that both companies rely on "the quality of reporting and accuracy" for ultimate business success. In addition, he said, the necessary independence of communications media owned by the Post Co. is similar to the independence D&B required for a subsidiary such as Moody's, whose credit and financial analyses of corporations and municipal governments have an established credibility in financial markets.

Simmons, a graduate of Harvard College and Columbia University's law school, joined D&B in 1969 after working for Southeastern Public Service Co. He became vice president and senior counsel of D&B in 1970 and subsequently served as an executive with the Reuben H. Donnelley advertising subsidiary and as president of Moody's and of the parent company's D&B unit.

In 1976, Simmons was named executive vice president of D&B Corp. and in 1979 he became a vice chairman and director. He currently shares in overall operating management responsibility for D&B and oversees its information-related divisions.

Simmons emphasized yesterday his view that successful communications companies must pay attention to new technologies that will bring more and more information to people at their offices and homes. "I think the challenge for the Washington Post Co., or any company engaged in information-collecting and dissemination, is to react and take advantage of the new technology which is all around us and changing at a dazzling rate," he said.

Companies that will be communications leaders 10 years from now are those that will "do the best job in relating new technologies to information needs of the people . . . a proper merging of those two will spell success," he said.

At the same time, Simmons expressed "an absolutely firm belief in the fact that the print medium is not going to disappear. . . . It's much more useful for comparisons . . . for associating ideas and finding answers to questions you don't know how to ask a [home] computer."

Post Co. Chairman Graham said Simmons would take over management of the Washington company's operations, which include The Washington Post and newspapers in Trenton, N.J., and Everett, Wash.; four television stations and a TV programming unit; and Newsweek and Inside Sports magazines.

"With his strong background in the information business, he has a real understanding of the markets we are in as well as those we expect to enter in the near future," Graham said of the D&B executive.

Both Simmons and a Post Co. official declined yesterday to reveal the new president's salary.

In an unrealted publishing industry development, Joseph Dionne was named president and chief operating officer of McGraw-Hill Inc. He succeeds Harold McGraw Jr.