A federal court jury in Washington yesterday awarded $1 million in damages to a 6-year old Vietnamese orphan for injuries caused in the crash of a jumbo C5A jetliner on a mercy mission out of Saigon in April 1975.

The girl, Melissa Hope Marchetti, is now a kindergarten pupil in Derby, Conn., where she lives with her adoptive parents. She was one of more than 100 infants who were huddled in seats in the huge aircraft's troop compartment when the plane crashed into a marshy rice paddy near Saigon and split up.

The jury of four women and two men deliberated for about 10 hours before announcing its verdict after a three-week trial before U.S. District Judge Louis F. Oberdorfer.

The four women who helped decide the Marchetti case yesterday also participated in verdicts in two other cases brought for infants who survived the crash. In one of those cases, the women and two other jurors awarded a 6-year old Denver boy $500,000. In the second case, however, those same six jurors decided against awarding any money damages to another orphan who now lives with hisadoptive parents in Missouri.

Various observers said yesterday that Marchetti's injuries were more serious than the injuries alleged in the two other cases. Moreover, the jury found yesterday that the girl's illnesses were the direct result of the airplane accident.

In the first case, in which the jurors awarded the Denver boy $500,000, the jurors found that the accident aggravated illnesses that the boy suffered prior to the crash.

Individual lawsuits have been filed in the federal court here in behalf of 67 of the 150 infants who survived the disaster. The defendants in the suits are Lockheed Aircraft Corp., which manufactured the plane, and the United States government, which had acquired the plane for use by the Air Force. Lockheed and the federal government will share the cost of the damage awards under terms of an agreement.

Of the 330 persons aboard the ill-fated flight, 135 were killed, including 76 orphans, all of whom were on their way to new homes in the United States and Europe.

Lawyers for Marchetti had argued that she suffered brain injury from the sudden loss of oxygen and trauma during the crash, leaving her with a range of permanent emotional and learning disabilities, including hyperactivity.

All the orphans have been represented at each trial by Oren R. Lewis Jr. and lawyers from his Arlington law firm.

Lockheed's principal lawyer, Carroll E. Dubuc, had contended that the girl's symptoms were due to poor conditions in Vietnam, malnutrition of her natural mother and later of the child herself, and other physical or mental illnesses the infant experienced during the first year of her life.

The girl's adoptive mother, Pamela Marchetti, 31, who was present when the jury returned its verdict, said that the jury's decision meant that "if the worst ever came through," in terms of the doctor's predictions for her daughter's condition "the money would be there to take care of her properly."

Marchetti said that her husband, Dennis, who served in the Marine Corps in Vietnam during the war, had always wanted to adopt one of the children he had seen there. She said yesterday that despite the plane crash, the later ordeal of the court trial and Melissa's uncertain future, "It's all worth it, Melissa's worth it."

As in most aviation disaster cases, it had been expected that lawyers on both sides would use the early jury verdicts as a gauge to reach out-of-court settlements in many of the remaining cases.

Judge Oberdorfer, however, in a memorandum released yesterday, noted that both sides are on a "collision course" and that any "early settlement of the claims is unlikely.

If trials were held for each of the 147 remaining surviving infants, Oberdorfer said, the federal court could be tied up with these cases for at least six years.

Oberdorfer said that a long series of jury trials "may be too crude an instrument to achieve justice" in the infants' cases. He urged both sides to consider a long-range plan for evaluation and treatment of the children until their cases come to trial.

One difficulty with the cases, Oberdorfer said in his memo yesterday, is that some of the children may win big jury verdicts but then never develop difficulties forecast during the trial. The result, the judge said, is that Lockheed stockholders and federal taxpayers will assume the cost of a "windfall" for the infant, its family and its lawyers.

In some cases, Oberdorfer said, the infants symptoms may be so vague at the time of trial that the jury may award no damages at all. The upshot, Oberdorfer said, is that those children whose cases are carried in later years, when their symptoms have "ripened," will have an advantage over children whose cases are tried earlier before their injuries fully manifest themselves.

Basically, Oberdorfer suggested that both sides agree to a plan to try the remaining cases as each child's symptons reach the stage at which they can be fairly assessed.