Former Virginia Attorney General J. Marshall Coleman, in one of his last official acts, decided two weeks ago not to prosecute a former highway commissioner whom Coleman's staff had investigated for alleged conflicts of interest.

Coleman, the unsuccessful Republican candidate for governor last fall, ended a criminal investigation of former commissioner T. Ray Hassell III of Chesapeake, also a Republican, the day before he left office. A spokesman for Coleman's Democratic successor, Gerald L. Baliles, said yesterday Baliles has no plans to reopen the case.

During a three-month investigation, Coleman's staff found that Hassell had lobbied intensively and secured highway funds for a Chesapeake road that his engineering firm was designing. Hassell actively pushed the project, sometimes convincing his nine fellow commissioners to bend policy in order to win quick approval for the work, The Washington Post reported last month.

Hassell was forced to resign from the commission in October by Gov. John N. Dalton for what the governor called "technical violations" of the conflict of interest law on another issue. Hassell, a land surveyor, had failed to disclose his highway post when his engineering firm applied for other government contracts.

Coleman, now practicing law in Washington, said yesterday that he decided against prosecution on the recommendation of James Kulp, the head of his staff's criminal division. Kulp, who has been retained by Baliles, could not be reached for comment, but an official in the attorney general's office confirmed Coleman's account.

That official said that inconclusive affidavits taken from chief Virginia highway commissioner Harold King and one of his assistants persuaded Kulp that Hassell could not be prosecuted successfully. King said in his sworn statement that he remembered in a general way Hassell discussing the road project, but could recall no specific incidents, according to the official.

The two highway officials also said they remembered Hassell discussing the project more from the perspective of Chesapeake's need for roads than as a way to benefit his firm, the official said.

King could not be reached for comment yesterday. Hassell, who has declined to discuss the specifics of the case, said he had not heard from the attorney general's office but would not be surprised by a decision to drop the case. "There's nothing to prosecute me for," Hassell said.

Highway department files show that Hassell actively promoted the Chesapeake project, an extension of Cook Boulevard, despite a warning he received from Deputy State Attorney General Walter A. McFarlane in October 1978. McFarlane told Hassell in a letter that under the conflict law neither he nor his firm could participate in any public contracts that received money directly or indirectly from the highway department.

Hassell's firm designed the Cook Boulevard extension for the Chesapeake Industrial Authority, which owns an industrial park through which the road runs. During his two-year lobbying effort for the road, Hassell's firm received checks from the authority totaling $62,000 for work on it and other projects.

Hassell met with highway officials several times to keep the project on track when funding problems threatened to delay it, highway records show. At least two officials complained in official memos that normal procedures and policies were being ignored.

Coleman would not discuss details of his decision yesterday. "My staff recommendation was against prosecution," he said. "Of course, the new attorney general would be within his rights to decide otherwise."

Hassell was one of three highway commissioners to resign last fall because of violations of the conflict of interest law. William B. Wrench, the Northern Virginia commissioner, resigned in September after it was revealed that he voted to locate the proposed Springfield Bypass near property he owns in Fairfax County. Winchester commissioner H. Delmer Robinson Jr. resigned when it was disclosed that two firms of which he was president and vice president, respectively, had done business with the highway department during Robinson's tenure on the commission.