A heat sink assembly includes a heat sink and a clip assembly received in the heat sink. The clip assembly comprises a clip and a movable fastener pivotally connected to the clip via a pair of supporters. Each supporter defines a pivot hole deviated from a center thereof and a retaining slot above the pivot hole. The clip comprises a main body received in the heat sink and two arms extending from the main body and pivotally received in the retaining slots of the supporters. The movable fastener pivotally extends in the pivot holes of the supporters. The movable fastener moves relative to the heat sink and causes rotation of the supporters in a matter such that a distance from the main body of the clip to a bottom of the heat sink is changed, whereby the clip assembly can provide adjustable spring force acting on the heat sink.