Muriel Buck Humphrey took the oath of office as U.S. senator yesterday and promised to help pass the legislative program left behind at the death of her husband, Hubert.

"I hope I can do as well," she said. "I hope I can fill Hubert's shoes."

She told reporters after the brief ceremony on the Senate floor that she has not decided whether to run in a special election to be held 10 months from now.

She said she will make that decision within two months and added. "I'll try not to keep everybody hanging."

Mrs. Humphrey, wearing a powder blue suit, waved to family members and friends in the Senate visitors' gallery and replied with a quiet, "I do," when Vice President Mondale administered the oath of office.

Senators, staff members and those in the gallery stood and applauded the new senator. It was the first standing ovation in the Senate since Humphrey himself returned to Washington from Minnesota last Oct. 25 after his doctors at home had decided the cancer from which he suffered as inoperable.

Mrs. Humphrey, who will be 66 on Feb. 20, was welcomed to the Senate by most of its members with hand-shakes and, more often, kisses.

Mrs. Humphrey's appointment makes Minnesota the only state with two appointed senators and a non-elected governor.