Sen. Frank Church (D-Idaho), who is expected to assume the chairmanship of the Senate Foreign Relations Committee in the new Congress, has selected William Bader, a Pentagon official, to be the committee's staff director, informed sources report.

Bader, 47, is deputy director of the office of planning for the secretary of defense. He has been responsible for coordinating defense intelligence operations.

Bader served on the staff of the Foreign Relations Committee from 1966 to 1969. His appointment is not likely to be anounced until Church has been formally installed as chairman in January.

Bader would succeed Norvill Jones, who is reported to be taking a job in private industry.

Staff members on Foreign Relations report directly to individual senators who, in effect, named them to their positions, so the staff director is less powerful than on many other committees. But as the chairman's right-hand man, he can still have substantial influence. Informed sources speculated that Bader could play an important role in the Senate debate on a new strategic arms limitation treaty (SALT) with the Soviet Union. As chairman of the committee, Church is expected to be the SALT floor leader.

Bader, with a doctorate from Princeton, is a former foreign service officer and official of the Ford Foundation. He worked for Sen. Church on the staff of the select committee that investigated the Central Intelligence Agency.