Chief Alexandria Circuit Court Judge Franklin P. Backus said yesterday he plans to retire from the bench on Feb. 1 after 26 years of service. His announcement set off speculation as to a possible successor.

The two candidates most frequently mentioned to fill the forthcoming vacancy are former Virginia House of Delegates majority leader James M. Thomson and attorney Albert H. Grenadier, both Democrats. Thomson said yesterday he is actively interested in the position, which pays $42,500 annually, and had begun trying to line up support. Grenadier could not be reached for comment.

Roger L. Amole, president of the Alexandria Bar Association, said yesterday his organization will hold an endorsement meeting in January to recommend a successor to Backus, 64.

Since the resignation will become effective during a session of the General Assembly, the Democrat-dominated legislature will elect Backus' successor. Customarily the person endorsed by the local bar association is assured of election. Had Backus made his resignation effective during a time when the legislature was not in session, Gov. John N. Dalton, a Republican, would have appointed a successor, subject to approval by legislators.

Backus said yesterday that age was a principal factor in his decision to resign. "I will be 65 years old on Dec. 22, and I think it is time to move on. I don't know what I'm going to do, because I have never been retired before," he said.

Backus served as a member of the Alexandria City Council and as mayor before being named a judge of the old Corporation Court in 1953. That court heard cases from both Alexandria and Fairfax County; it was reorganized in 1973 to become exclusively a city court and was renamed the Circuit Court.

The Alexandria Circuit Court now has three judges, Backus, Donald Kent, and Wiley Wright.

Wright has been named a candidate for one of the 117 new U.S. District Court judgeships created recently by President Carter. If Wright ultimately is chosen over four other local candidates for the new Northern Virginia seat, his appointment would open up another vacancy on the Alexandria court.