About 70 members of the International Alliance of Theatrical Stage Employees picketed the show Saturday at Hollywood Center Studios, prompting producers to cancel a taping and dismiss a studio audience.
IATSE official Vanessa Holtgrew told the Hollywood Reporter the union was striking against the show until a contract is reached.
The union allows members to work on the non-union "Fashion Star," but they get no pension and health benefits without a union contract.
IATSE is fighting to keep a foothold as union-heavy productions like feature films in Los Angeles increasingly give way to productions that tend to be non-union like reality shows.
NBC and the show's three production companies had no immediate comment.
(Copyright 2012 by The Associated Press. All Rights Reserved.)