A Federal jury found today that an air traffic controller and the pilot of a private plane were equally responsible for a midair collision with an Aeromexico jetliner that killed 82 people here three years ago.

The verdict could leave the Government and the estate of the private plane's pilot, William Kramer, liable to pay millions of dollars in damages to relatives of victims of the collision, which occurred over the Los Angeles suburb of Cerritos. The actual amount of damages will be determined at later trials or through settlements.

The plaintiffs, all relatives of victims, had sued Mr. Kramer's estate, the Federal Aviation Administration and Aeromexico. Of the three defendants, the only one to which the jury did not assign blame today was Aeromexico, whose DC-9 collided with Mr. Kramer's Piper Archer after the smaller plane had strayed into restricted airspace. All 64 people aboard the Aeromexico jet were killed in the accident, as were Mr. Kramer, his 2 passengers and 15 people on the ground.

The jury's findings roughly paralleled those of investigators for the National Transportation Safety Board who looked into the crash. The jurors told reporters after the verdict that while they believed Mr. Kramer had acted negligently, the Government was also to blame because the air traffic controller, Walter White, should have steered the jetliner away from the smaller plane.