Robert H. Bork, solicitor general in the Nixon administration, will be nominated by President Reagan to the U.S. Court of Appeals for the District of Columbia, sources in both the administration and legal profession said yesterday.

Bork was a leading candidate for the Supreme Court seat to which Reagan nominated Arizona Judge Sandra D. O'Connor, and the pending nomination is seen as a strong indication he will remain a contender for the next vacancy on the nation's highest court.

His appointment would put a strong conservative on the powerful appellate court here, which has been described as second only to the Supreme Court in influence. The court now has 11 full-time members. It has long had a liberal reputation, and the four most recent appointees, all of whom were put forward by President Carter, are regarded as liberals.

Presidential counsel Fred Fielding confirmed that "a tentative decision has been made" for a successor to Judge Carl McGowan, who announced last month his decision to retire from fulltime service Aug. 31. A formal announcement of the appointment will await the completion of clearances, which might take several weeks, Fielding said.

But other sources confirmed that Bork, a widely respected constitutional scholar, is the administration's choice.

One Justice Department official involved in the selection process described Bork's pending appointment as part of an administration design to "turn the circuits around on such issues as budget, taxes and regulatory reform."

The official, noting that Carter had appointed 40 percent of all the sitting circuit judges in the country, said, "We've got very few vacancies on the circuit courts. They have got to be very carefully chosen."

Bork would be the third recent appellate nominee to meet the administration's criteria, the official said. The other two are Ralph Winter, a Yale University professor selected for the 2nd Circuit Court of Appeals in New York, and Richard Posner, a University of Chicago professor selected for the 7th Circuit in Chicago.

"All three are well-renowned, highly respected non-judges with strong economic backgrounds," the official said.

Bork, who left Yale Law School recently to join the Washington law firm of Kirkland & Ellis, is an antitrust expert known for his strong views against federal regulation of business mergers.

He is the author of Antitrust Paradox, which argues that government antitrust policies have put unreasonable constraints on production and economic growth.

Yale Law School Dean Harry H. Wellington said that Bork "is concerned about what courts are able to do and not able to do in overextending themselves, in taking on things that are better handled by the legislative process."

Bork, however, also has spoken against stripping courts of jurisdiction.

"He approaches legal matters on a case-by-case basis," Wellington said, "like a good lawyer."

Observers believe Bork, who is respected for his intellect and thoughtful approach to legal problems, could become a powerful counterweight to the young, liberal wing of the appeals court, and may be able to bring to his side some court members who would not normally join forces with the conservative members.

"He will be somebody to reckon with," one court source said.

One lawyer commented that Bork may find himself "starved to death" for his favor topic -- antitrust law -- in the D.C. Circuit, where the diet is heavily weighted with administrative law cases.

Bork is not known as a judicial activist, and would be expected to carry that philosophy through in the administrative law area. The Supreme Court has reprimanded the D.C. circuit for going too far in attempts to resolve some administrative law cases.

Bork achieved a certain notoriety in the Watergate era when, on Oct. 20, 1973, he carried out President Nixon's order to fire Watergate special prosecutor Archibald Cox. In what became known as the "Saturday Night Massacre," Attorney General Elliot L. Richardson and his deputy, William D. Ruckelshaus, resigned rather than dismiss Cox.

Two months ago, Cox and Bork met again, at a Senate Judiciary subcommittee hearing where they agreed that a bill declaring that human life starts with conception probably is unconstitutional.

Staff writers Charles R. Babcock and Laura A. Kiernan contributed to this article.