The Washington Post asked the full U.S. Court of Appeals here yesterday to reconsider a 2-to-1 decision last month that reinstated a lower court jury's decision that the newspaper libeled former Mobil Oil Corp. president William P. Tavoulareas in a 1979 article about his business dealings.

The newspaper, in asking for the rehearing, said the decision, if left to stand, would "radically transform the law of libel" and that "investigative reporting undoubtedly will be discouraged."

In last month's ruling, Senior Circuit Judge George E. MacKinnon, in an opinion concurred in by Judge Antonin Scalia, said that a jury could find actual malice on The Post's part by considering evidence that the newspaper "seeks, among other things, hard-hitting investigative stories." Judge J. Skelly Wright dissented.

Under a 1964 Supreme Court libel decision, newspapers are guilty of actual malice when they knowingly print falsehoods or publish material with a reckless disregard for the truth. The newspaper said the two-judge appellate majority "effectively redefines actual malice" by allowing a determination to be based on a "vague 'cumulation' of considerations that are irrelevant and improper."

For The Post to win a rehearing, at least six of the 10 active appellate judges would have to agree to reconsider the case. If the case is reheard, MacKinnon, since he participated in the three-judge panel's decision, would also be included in the 11-judge panel.

The appellate decision also reinstated the U.S. District Court jury's $2,050,000 damage award to Tavoulareas, which U.S. District Judge Oliver Gasch had overturned.