SAN DIEGO (AP) _ Navy investigators have concluded that mechanical failure caused the crash of a fighter jet last year in which a highly decorated pilot was killed, The San Diego Union reported today.

The cause of the Dec. 2 crash of the F/A-18 Hornet is outlined in a report to be sent to the Pentagon next week, the newspaper said. It said the report concluded a mechanical failure in the landing gear led to the crash at the Miramar Naval Air Station.

Killed in the crash was Capt. Henry Kleemann, who made headlines in 1981 when he led the downing of two Libyan warplanes over the Gulf of Sidra. Kleemann's jet skidded off a wet runway, flipped over and pinned him.

The Union quoted unidentifed sources as saying Kleemann falsified his own flight records in order to fly the sophisticated F/A-18. Navy sources said Kleemann had completed 50 hours in the jet's cockpit with instructors and would have been qualfiied to fly it after a few more flghts.

The sources said, however, that the mechanical failure and not Kleemann's inexperience led to the fatal crash.

Navy investigators focused on the landing gear's planning link, a 22-inch- long guide rail that keeps the jet's nose wheel straight during landings. When the link failed, investigators believe, the nose wheel was allowed to spin.

Tim Beecher, a spokesman for McDonnell Douglas Corp. of St. Louis, which builds the Hornets, said the company would study the aircraft as a matter of routine in such cases.

Cmdr. Tom Jurkowsky, spokesman for Pacific Fleet Air Forces, said pilots who have flown the Hornets swear by their performance and said the jets' handling characteristics ''are exemplary - in fact, exceeding that of other aircraft in the Navy's inventory.''

Five other F/A-18 Hornets were involved in landing accidents in 1985. Navy officials who investigated the other incidents decided that three were caused by pilot error. The causes of the other accidents have not been determined.

All the 1985 accidents occurred on wet runways or in bad or marginal weather. In each, the jet went out of control and skidded during landing. There were no other deaths.