Harold H. Velde, 75, an Illinois Republican who served in the House of Representatives from 1949 to 1957, including a term as chairman of the old Committee on Un-American Activities, died Sept. 1 at a hospital in Sun City, Ariz. The cause of death was not reported.

As chairman of HUAC from 1953 to 1955, he attempted to investigate nearly everything for possible Communist subversion or infiltration. He tried to force former president Harry S Truman to testify before the committee, but Truman refused. Mr. Velde wanted the Missouri Democrat to explain alleged lapses in security during his administration, but, citing the separation of powers, Truman declined to testify.

Mr. Velde angered educators while searching vainly for communist conspiracies on campus. His comments on the desirability of investigating the nation's churches and clergy produced a greater degree of interdenominational unanimity in opposition to him than clergymen generally demonstrate.

His offer to serve as watchdog against what he said would be a major Soviet attempt to infiltrate the Republican Party in the 1950s was appreciated by few of his fellow party members.

He also was forced to admit that Agnes Meyer (a writer and the wife of former Washington Post publisher Eugene Meyer) was not the Soviet sympathizer he had charged her with being. He said the error stemmed from a mistranslation of Russian documents into English.

After facing bipartisan rebellion on his committee and surviving serious challenges in 1954 in both the primary and general elections in Illinois, he announced that he would not run in 1956. After leaving the House, he practiced law for a time in Washington and Illinois and in 1969 was named a regional counsel with the General Services Administration.

Mr. Velde, who lived in Arizona since 1974, was born in Parkland, Ill. He was a 1931 graduate of Northwestern University. He was a high school teacher and coach before earning a law degree at the University of Illinois in 1937.

During World War II, he served with the Army Signal Corps and as a special agent with the FBI. After the war, he returned to Illinois and was county judge of Tazewell County until winning election to the House of Representatives in 1948.

He was named to HUAC in January 1949. He became chairman when Rep. J. Parnell Thomas (R-N.J.) was sentenced to prison after being convicted of accepting kickbacks from his office staff.

Mr. Velde's first wife, the former Olive Pfander, whom he married in 1931, died in 1952. In 1954, he married the former Dolores B. Harrington. They were divorced in 1956 and remarried the following year. Survivors include his wife, of the home; two children by his first marriage, and two stepchildren.