U.S. Court of Appeals Judge Malcolm R. Wilkey, a conservative appointed to the appellate bench in 1970 by President Nixon, said yesterday that he will retire to part-time status next month.

Wilkey said the move to so-called senior status will take effect on Dec. 6, his 66th birthday. Senior federal judges carry a reduced workload, although they keep their offices, staff and full salary.

Wilkey's departure will leave the prestigious 12-member U.S. Circuit Court here with two vacancies to be filled by the White House. President Reagan's nominee for the other seat, Deputy Solicitor General Paul M. Bator, withdrew recently for health reasons.

The loss of Wilkey, a former assistant attorney general under President Eisenhower, is not expected to shift the ideological balance of the court, which is generally considered to be 6 to 5 with liberals in the majority. Reagan presumably would fill the vacancy with another conservative.

Wilkey said he notified the president of his decision Tuesday by letter. He said he was motivated to make the move by the "wear and tear of the job" and believed it was time to "shift my emphasis away from full blast."

The judge said he will leave Washington in March to spend nine months as a fellow at Cambridge University in England. After his return, he said, he plans to settle in Houston, where he served as U.S. attorney in the late 1950s.

He said he expected to continue to hear cases here and with the 5th U.S. Circuit Court of Appeals, which is based in New Orleans. A former general counsel for Kennecott Copper Corp., Wilkey was sworn in as a federal appeals court judge on March 30, 1970, filling a vacancy left by Chief Justice Warren E. Burger.