Virginia Gov. Charles S. Robb plans to remove Maj. Gen. William J. McCaddin, the longtime head of the Virginia National Guard, according to several legislators.

"I heard that the governor said he was not going to reappoint him," said Del. Frank Slayton (D-South Boston), adding that he expects a decision on the prestigious job as adjutant general later this week. McCaddin has held the job since 1970, serving under three successive Republican governors.

State Sen. Edward E. Willey (D-Richmond) said he had been told at the start of the Robb administration early this year that McCaddin would not be reappointed.

Secretary of Transportation Andrew Fogarty then asked legislators to submit names of candidates to head the Guard, Willey said.

George Stoddart, Robb's press secretary, denied yesterday that a decision had been made to replace McCaddin. "He's one of several candidates," Stoddart said. "It's still under consideration."

McCaddin's appointment has been renewed twice since 1970; last January, when his latest term expired, Robb allowed him to continue in a holdover position.

Slayton said yesterday that Robb last winter tested the idea of appointing Lt. Gen. Samuel V. Wilson, a former intelligence official and non-Guard member, to the post.

The plan was dropped after Robb learned of legislative opposition to the required change in the law that would have allowed a non-Guard member to assume the adjutant general job, Slayton said.

At least two studies of the Virginia National Guard suggested that the program's management needed improvement, Slayton said.

One, done by the National Guard Bureau, questioned the Guard's readiness; another, by Gov. John N. Dalton's administraton, suggested improvements in the fiscal management of the agency, according to the South Boston legislator.

A third study of the guard was done this year by Gen. Wilson, but Slayton said he did not know the nature of Wilson's recommendations.

Several candidates have been mentioned for the post, including Brig. Gen. John G. Castles of Fredericksburg, who is being promoted by Slayton, and Col. James E. Tyler of Richmond, backed by Willey.