The American University's board of trustees yesterday elected Richard Berendzen successor to former university president Joseph J. Sisco, who resigned to assume the part-time position of chancellor.

Berendzen, currently AU provost, will take office next Jan. 13, the day Sisco leaves to become chancellor.

Sisco, former undersecretary for political affairs in the State Department, became American University president in July 1976 after a 25-year diplomatic career.

He said yesterday he wanted more time for speaking and writing about foreign affairs. Under the arrangement agreed to with the board of trustees, Sisco will spend about half his time writing and speaking and about half as AU chancellor. Among the major duties of that office will be fund raising and bringing American University to the public's attention.

Methodist Bishop James K. Mathews, chairman of the board of trustees, called the shift "what amounts to a reorganization of our leadership team."

Berendzen, 40, has been physics professor and provost - the top academic office - at American University since 1976. Before that he was dean of the college of arts and sciences. He earned his doctorate at Harvard and has taught at Harvard and at Boston University.

With an enrollment of about 12,500 American University has made a major effort to raise academic standards in the last few years, an effor that Berendzen has been heavily involved in as provost and one that he said will continue.