A newly discovered diary kept by President Harry S. Truman in 1947 tells of a private conversation in which he urged Dwight D. Eisenhower to seek the Democratic presidential nomination the next year, with Truman as his vice-presidential running mate, the National Archives announced today.

Although there was speculation of such a plan at the time, Truman always denied it. The diary, however, shows that he was concerned that Gen. Douglas MacArthur, then the military governor of Japan, might win the Republican nomination. So he had the idea of countering with another popular World War II hero, Eisenhower.

''Ike and I think MacArthur expects to make a Roman triumphal return to the U.S. a short time before the Republican convention meets in Philadelphia,'' says the relevant entry, dated July 21, 1947. ''I told Ike that if he did that,'' then Eisenhower should ''announce for the nomination for president on the Democratic ticket and that I'd be glad to be in second place, or vice president.''

In the end, MacArthur's Republican supporters failed to win the nomination for him, Truman won the Democratic nomination and defeated Thomas E. Dewey in the general election, and four years later Eisenhower became Truman's successor, as a Republican.