A lack of firm evidence indicating that John W. Hinckley Jr. was actually sane, as the prosecution contended, was cited last night by one of the jurors in his trial as a main reason for their not guilty verdict.

"There was not enough evidence that he was sane," juror Virginia N. Smith told a reporter from her brick house in the middle-class Woodridge section of Northeast Washington.

"All the psychiatrists found that there was a mental disorder," Smith said, in reviewing the six weeks of testimony heard during the highly publicized trial.

"The defense psychiatrists, the prosecution psychiatrists all found a mental disorder," the 61-year-old juror said, " . . . it was the consensus of all the psychiatrists that there was a mental disorder."

In discussing her role and that of her fellow jurors in the trial, which had been seen as a crucial test of the insanity defense, Smith said, "You have to go on the evidence, not your personal feelings."

Smith, listed as a retired government employe, said she believed Hinckley "was a disturbed person mentally." Only the degree of his disturbance was in dispute, she said.

None of the jurors reached by The Post discussed the process used by the jury during the 24 hours of deliberations, conducted over four days, that led to the verdict that Hinckley was not guilty because he was legally insane when he shot President Reagan and three others.

But it appeared the intensity of deliberations reached a peak yesterday. Contacted by telephone, the mother of juror Lawrence H. Coffey said her son looked weary and drained when he returned last night to their Anacostia home.

" 'Mama, you just don't know what we went through today,' " she quoted her 22-year-old son as saying.