The D.C. Court of Appeals ruled yesterday that a conviction should be overturned on the basis of a lawyer's poor work at the appeal stage only if the representation is grossly inadequate.

The court, in a 34-page opinion written by Senior Judge George R. Gallagher, said a conviction may be thrown out on the basis of ineffective assistance of counsel only if the lawyer on appeal failed even to make an argument that -- if raised -- would have created "the reasonable probability of reversal."

The court let stand the first-degree murder conviction of Curtis L. Watson, who was also found guilty of assault charges stemming from unrelated incidents. Watson claimed that his lawyer on appeal failed to adequately challenge the trial judge's decision to try the assault case with the murder charge.

The defendant's Sixth Amendment right to effective legal help is not violated if the lawyer made the argument but simply argued it badly, Gallagher said in an opinion joined by Judges Frank Q. Nebeker and Julia Cooper Mack.

The case is one of the first in the country to set out the proper standard for reviewing a lawyer's appellate performance in the wake of a 1985 U.S. Supreme Court ruling that the constitutional right to effective assistance of counsel extends to a defendant's first appeal.

The court set a lower standard for lawyers' work on appeal than at the trial stage because, it said, "ineffective representation at the trial stage is apt to have a more telling effect on a defendant's case than poor representation on appeal . . . . It is the unusual appeal whose outcome depends upon exceptional skill of counsel because even a poor appellate argument is met with incisive consideration by the appellate court."

James Klein of the Public Defender Service, which represented Watson in the appeal case decided yesterday, said he would ask the full court to review the decision.