Sept-Îles judge Michel Parent ruled today that Gauthier threatened entrepreneur Frédéric Boucher on a job site in June of last year.
During a work strike in 2013, many union reps patrolled construction sites to make sure they were not in operation.
Parent said that Gauthier uttered violent threats to Boucher to force him to stop working.
The contractor testified he was only doing stone crushing work, an activity that is not considered illegal during a strike.
Bernard Gauthier could face up to six months in prison and a fine.
FTQ-Construction spokesman Merlin Trottier-Picard declined to comment since sentencing has not taken place yet.
Gauthier will be sentenced on Feb. 3, 2015.