The Labor Department's chief of staff, Daniel K. Benjamin, has been on leave for several weeks during what sources describe as an inspector general's investigation into possible improprieties.

Investigators are examining Benjamin's use of a lobbyist's boat for a sailing trip last summer, the sources said, as well as Benjamin's role in a noncompetitive contract awarded to one of his former research assistants, who also was on the sailing trip.

Benjamin's absence comes as Labor Secretary Raymond J. Donovan is on paid leave to fight his indictment on fraud and larceny charges in connection with a New York City subway contract handled by Donovan's construction company. Undersecretary Ford B. Ford is acting secretary.

The Labor Department's chief spokesman, Michael Volpe, said he could not disclose what kind of leave Benjamin has been taking since mid-October, but that Benjamin is continuing to receive his $66,400 annual salary. While declining to confirm or deny the investigation, Volpe said Benjamin had planned to take leave during this period. He said he does not know when Benjamin will return to work.

The Benjamin investigation was reported by the Associated Press.

Benjamin, 37, a former economics professor at the University of Washington, joined the Labor Department in 1982 and held several senior positions before becoming chief of staff last May. Benjamin, one of Donovan's closest aides and a key figure in managing the department, did not return calls to his McLean home yesterday.

Sources said the probe by Inspector General James B. Hyland began after the allegations were first reported by columnist Jack Anderson, and is focusing on these areas:

*Benjamin's use of a 32-foot boat owned by James McKevitt, a lobbyist for the National Federation of Independent Business. The group represents more than 500,000 small firms that are affected by Labor Department regulations and has been particularly interested in the administration's proposal for a subminimum wage for teen-agers.

After taking Benjamin for a trip on his boat, Navajo Chief, McKevitt allowed him to borrow the sloop for a weekend cruise in July.

Volpe said yesterday that Benjamin had spent several days helping to repair McKevitt's boat, and that "there's no law that says you can't work on a friend's boat and then take it out." He said he rejected "the inference that McKevitt was trying to get to the Labor Department through Benjamin," saying that McKevitt has known Donovan for four years, but met Benjamin only a few months ago.

Benjamin told the Associated Press last month that he and McKevitt rarely discussed official business and that he sees nothing wrong with his use of McKevitt's boat. "I think that in terms of conflict of interest, that's only if you determine there's a bad motive, and that's not the case," Benjamin said. "He and I are friends."

*Benjamin's role in a $2,357 contract awarded without bidding to Keith Wollenberg, who was Benjamin's research assistant at the University of Washington. Benjamin then was head of the division that hired Wollenberg.

Wollenberg's contract, which included his air fare from California, was to analyze public comment on ethylene dibromide (EDB), a hazardous substance regulated by the department. The seven-day assignment immediately followed the sailing trip on which Wollenberg accompanied Benjamin and Eric Sonett, a former student of Benjamin's who worked at the Labor Department last summer.

In a message to Sonett on a department internal computer system, Benjamin asked, "When is Wollenberg coming? Will he need money to finance the research he will be doing?"

In another message, Benjamin, whose computer code name was "captain," said, "Are you interested in helping me crew? . . . If KW can get in the night before, he can help too."

Volpe said that Wollenberg had experience studying EDB and that the contract had been approved by several career employes. Volpe said that "it is not unusual" for an official to hire "someone he knows and is familiar with the subject matter."

*Questions about documents Benjamin filed with the department that contain conflicting accounts of how much money he earned before joining the government.

Benjamin told The Washington Post last month that one of the documents did not include consulting fees he earned in addition to his college salary, and that one of his financial disclosure forms had been lost by Labor officials. He said he had filed an amended form to clear up any discrepancies.

Volpe said that Benjamin's disclosure forms were approved by career officials and that they had no bearing on his Labor Department salary.