Supreme Court Justice William J. Brennan Jr. is giving serious consideration to retiring and will make his final decision in December, according to informed sources.

Brennan, 73, told a reunion of nearly 60 of his former law clerks here Saturday night that if he decides to retire -- a course several close associates said is more than likely -- he would wait until the close of the court term next June to formally step down.

But he has indicated that he might announce his intention to leave before then, perhaps late this year.

Brennan is the senior associate justice, having served on the high court since 1956. He received treatment for a cancerous tumor in his throat early last year but is reportedly now in good health.

He did not tell his former law clerks exactly why he might retire but has told other that his wife Marijorie's own ill health is a factor.

In a sometimes emotional dinner with his former clerks Saturday night at the International Club of Washington, 1800 K St NW, Brennan dwelled on his past 23 years on the court and spoke little of the future, the sources said.

Addressing his former law clerks, each of whom served as legal adviser for a year, Brennan gave a year-by-year review of each court term naming each former clerk and recalling the significant events and cases.

About 58 of his 65 former clerks attended the dinner, an unusually high number, the sources said. Nearly all the Supreme Court justices have an annual reunion dinner of former law clerks here in Washington.

"The whole tone of the evening was as if this was the last," said one who attended.

Should Brennan resign, President Carter would choose his replacement. An interesting political situation could envolve for a number reasons.

First, bringing a nominee before the Senate in a presidential election year is generally tricky. Secondly, the hearings on the nominee would be held by the Senate Judiciary Committee, which is chaired by Sen. Edward M. Kennedy (D-Mass.), a likely challenger to Carter for the Democratic presidential nomination.

At Saturday's dinner Brennan confirmed published reports that he has not yet hired his law clerks for the next court term scheduled to begin in October 1980 and said he would decide that, too, in December. Normally Brennan has selected his clerks more than a year in advance.