A positioning mechanism for reflectors in a scanner is capable of tool-less assembling and precise positioning. The mechanism mainly includes a carrier, some angular positioning members, and resilient arms. The carrier is a plate-like member for mounting an optical assembly. The angular positioning members are mounted on the carrier and formed with cutoffs of supporting surfaces. The resilient arms extend from the carrier to outer sides of the supporting surfaces of the angular positioning members. The clearance of the end of resilient arm to the supporting surface is less than the thickness of the reflector to position the reflector simply, easily and precisely.