A fixing device fastens a heat sink having a base on one of motherboards with different specifications. Each motherboard with a corresponding specification defines a plurality of extending holes therethrough. The fixing device comprises a plurality of slats each having an end thereof pivotally connecting to the base of the heat sink and the other end thereof defining an elongated slot for corresponding to one of the extending holes of the one of the motherboards, a back plate defining a plurality of mounting holes corresponding to the extending holes of the motherboards, and a plurality of fixing units extending through the slots of the slats, the extending holes of the one of the motherboards and corresponding mounting holes of the back plate to mount the heat sink on the one of the motherboards.