A clip for fixing a heat sink on a retaining bracket includes an elastic supporter, an operating member, a movable fastener and a fixing bar. Two ends of the elastic supporter have a connecting portion and a first buckle portion, respectively. The operating member has a resisting portion, a pivot portion and an operating bar. The pivot portion pivots to the connecting portion. The movable fastener installs on the resisting portion and the connecting portion, and includes two sliding slots, a resisting surface and a second buckle portion. The resisting portion has an arc surface for resisting against the resisting surface. The distance between the apex of the arc surface and the pivot portion is the largest distance between the arc surface and the pivot portion. When the clip is locked, the junction of the resisting portion and the resisting surface excludes the apex of the arc surface.