The Reagan administration has notified a Pentagon official of plans to dismiss him for allegedly disclosing that President Reagan's five-year plan to rearm America could cost $750 billion more than expected.

The official, John C. F. Tillson, manpower management director in the office of the assistant secretary of defense for management, has denied that he was the source of the information, which was published Jan. 8 in The Washington Post.

Pentagon officials were infuriated by the report. Deputy Defense Secretary Frank C. Carlucci ordered lie-detector tests for all those, including himself, who attended the Jan. 7 meeting of the Defense Resources Board, at which the figures were discussed.

Tillson was notified in a March 9 memo from his immediate superior, Assistant Secretary of Defense Lawrence J. Korb, that he faced removal because of "disclosure of official information to unauthorized persons" and "disregard of DOD procedures for the protection of classified information."

Korb said three polygraph tests administered to Tillson indicated "deception" in response to questions as to whether he had provided any of the information for the article by Post staff writer George C. Wilson.

Wilson reiterated yesterday what he told Secretary of Defense Caspar W. Weinberger in writing on Feb. 18: that Tillson was not the source of his story. Wilson said he was prepared to swear to that, if necessary.

"A man's word still ought to be worth something, even in 1982," Wilson wrote Weinberger. "And I give you my word John was in no way connected with the story I gathered and wrote . . . . An honorable man stands falsely accused."

Wilson also said that the estimate of a possible $750 billion overrun was not even classified. "Part of the report which formed the basis for my story and for the briefing of the Defense Resources Board was classified and part of it was not," he said. "My information was that the $750 billion figure was not classified and if it were, it should not have been. Since when does accountability about cost overruns help the Russians and not the taxpayers?"

Tillson's lawyer, James H. Heller, said his client had also been accused of disclosing unspecified "official information" in February to five congressional staffers and of discussing some of the information that ended up in the Wilson article with a former Pentagon superior, Robin B. Pirie Jr.

"We've submitted written statements from all six people denying this," Heller said.

Pentagon officials also accused Tillson, a West Point graduate and decorated Vietnam veteran, of telling investigators that he had discussed classified information in the past with persons not authorized to have that information. Tillson, his lawyer said, denies that and says the investigators either misunderstood or misheard him.

A decision in the case is expected shortly from Weinberger. If Tillson is dissatisfied with it, he can appeal to the Merit Systems Protection Board.