Anti-union activities by USAir during a Teamsters organizing drive unfairly tainted the election, the National Mediation Board said yesterday in an order that authorizes another election.

The Teamsters had lost their bid to represent workers in a ballot count announced in January.

''We're extremely disappointed,'' said David Shipley, a spokesman for Arlington-based USAir Group Inc. ''As far as we're concerned, we ran a perfectly legal election campaign.''

Teamsters officials were not immediately available for comment.

The Teamsters, who were trying to win the right to represent 8,000 workers who load airplanes at 180 airports nationwide, had charged that USAir managers called meetings with small groups of workers to discuss the union's campaign, put up anti-Teamsters posters urging workers to burn their ballots and produced two anti-Teamsters videotapes.

The National Mediation Board, which governs labor-management relations in the airline and railroad industries, found that separate actions by USAir were not by themselves improper but together amounted to unfair interference with the union's campaign to represent workers.

The Teamsters may request a rerun election within 30 days.

Another election means an all-or-nothing event for the Teamsters, who already represent about 1,800 USAir workers in Pittsburgh, Philadelphia, Buffalo and Boston. If the union loses again, those workers would no longer be Teamsters members.

The Teamsters had represented cargo workers at USAir before its 1989 merger with Piedmont airlines, whose employees were not members of the union. In December, the mediation board ordered the election to determine whether workers at USAir and former Piedmont employees would be represented by the Teamsters.