Peter Tarnoff, a former Foreign Service officer, was elected by the Council on Foreign Relations' board of directors yesterday to be president of the private research and study organization.

Tarnoff, 48, who was executive secretary of the State Department when Cyrus R. Vance was secretary of State, was the choice of the council board over Robert C. McFarlane, who was national security affairs adviser to President Reagan until early this month.

McFarlane had told reporters and the council's search committee in recent days that he did not want the job. A council board member who asked not to be identified said that McFarlane's name was put forward and backed in yesterday's meeting only by retired Lt. Gen. Brent Scowcroft, national security affairs adviser to President Gerald R. Ford. Scowcroft declined comment.

William D. Rogers, a Washington attorney and former undersecretary of State who headed the council's search committee for a new president, said Tarnoff was elected "by acclamation," although one member of the board, whom he declined to identify, was "not entirely in favor."

A council spokesman said about 16 members of the organization's 25-member board attended the 90-minute meeting in the New York office of former council chairman David Rockefeller.

Following the vote, Tarnoff, who was not present earlier, thanked the board for its action and shook hands with board members. Late in the morning, he met with staff members of the 64-year-old organization.

Rogers said Tarnoff was the choice of the search committee after intensive study of 75 people suggested for the job, which was vacated in June by Winston Lord, director of State Department policy planning for then-Secretary of State Henry A. Kissinger. Lord left the council job to become ambassador to the People's Republic of China.

Rogers, who was undersecretary of State for economic affairs under Kissinger and who is now a private attorney for him, said one of Tarnoff's qualifications was that he was "a spectacular success" during the past two years as executive secretary of the World Affairs Council of Northern California. This San Francisco-based organization is "virtually a carbon copy" of the New York-based council, Rogers said.

Tarnoff, in a telephone interview, said he is awaiting the report of a special committee on the "national role" of the Council on Foreign Relations before charting his course at the organization's helm. The special committee is headed by former Commerce secretary Peter G. Peterson, who succeeded Rockefeller in October as chairman of the council board of directors.

"We're going to be looking at ways to speak to a large number of interested and influential people beyond the eastern seaboard" as part of an expanded national role, Tarnoff said. Among ideas under discussion, he said, are television broadcasts and teleconference techniques to bring council programs to more areas of the country.

Tarnoff, a Foreign Service officer from 1961 to 1982, served in Nigeria, Vietnam, France, West Germany and Luxembourg as well as in Washington. He is married to Mathea Falco, who was assistant secretary of State for international narcotics matters in the Carter administration.