HANOVER, N.H.--Dartmouth College President John G. Kemeny announced Monday that he will resign August 1, 1981, or earlier if a successor is found before that date.

Kemeny said his decision to resign was based on "a realistic evaluation" of what further he could expect to accomplish as president. He said the "wear and tear of the job" had taken its toll on his energy.

Kemeny said he will return to Dartmouth to teach mathematics after a year's sabbatical. "The students are not Josing a president; they are gaining a full-time faculty member," he said.

The 54-year-old native of Hungary was chairman of Dartmouth's mathematics department for 12 years before rising to the presidency in 1970. He has served longer than any other current Ivy League President.

The board of trustees has formed a search committee which will take six months to a year before issuing a report, a university spokesman said yesterday.