WASHINGTON (AP) _ A federal court jury has awarded a free-lance reporter $200,000 because, the panel said, American Broadcasting Cos. did not give him ''on-air'' credit in the network's 1978 television stories based on an article he wrote about then-President Carter's finances.

The six-member jury awarded $150,000 in compensatory damages and $50,000 in punitive damages Wednesday night to Peter Peckarsky, who wrote ''The Buying of the President,'' concerning aspects of Carter's personal and campaign finances.

The network said it plans to appeal.

Peckarsky, a lawyer who researched the story for a year, charged ABC and several individuals with breach of contract and fraudulent and negligent mispresentation, among other things.

U.S. District Judge Harold H. Greene dismissed most of the charges against the network and all of them against individuals, except C. Robert Zelnick, then director of news coverage for the Washington bureau of ABC News.

Peckarsky charged that Zelnick promised to pay him $1,500 to work for three weeks in August 1978, and $1,000 each time ABC used his story on television and $100 on radio. He said ABC paid him only a total of $3,500 even though his article was used on nine telecasts and three radio newscasts.

He also contended that Zelnick promised to give him audio and video credit each time the article was used but did not.

Lawyers for ABC said the network paid Peckarsky $500 a week for three weeks while the article was being investigated. They also he was paid $2,000 for the initial two broadcasts, on ''World News Tonight'' on Oct. 19, 1978, and ''Good Morning America'' on Oct. 20, 1978.

The network argued that those were the only broadcasts based on Peckarsky's article, while the others were follow-up stories reporting on the reaction of the initial newscasts.

The ABC lawyers also denied that Zelnick promised to give him on-air credit because he lacked the authority to bind news programs to such an arrangement. Zelnick told Peckarsky he would try to obtain air credit for him but made it clear he could not commit the network to such an agreement, they said.

Peckarsky sought $150,000 on breach of contract, $100,000 compensatory damages for fraudulent misrepresentation and $10 million in punitive damages, and $100,000 in compensatory damages for negligent misrepresentation.

During the trial, Judge Greene dismissed the claim of negligent misrepresentation, which said Zelnick made the promise of on-air credit but should have known he did not have the authority to carry out the pledge.

The all-women jury found there was no breach of contract between ABC and Peckarsky.

The panel ruled, however, there was fraudulent misrepresentation because Zelnick promised him the on-air credit and knew he didn't have the authority to do so. It awarded the damages on that claim.

Peckarsky said he was happy not only for himself ''but for what this suit may do to protect the rights of other freelancers.''

Michael Calvey, ABC's lead attorney, said from New York that the network would file a motion, possibly as early as next week, asking Judge Greene either to set aside the award or to order a new trial.

''We think the jury misperceived one or more aspect of that (fraudulent misrepresentation) claim,'' he said. ''The evidence was totally contrary to the verdict.

''We hope our motion will be successful,'' said Calvey. ''We want to submit it as quickly as possible.''