A heat sink assembly includes a heat sink (80), a pair of fixing devices (40), a pair of clips (20), and a fan (90). Each clip includes a main body (24), and an operating arm (22) pivotally attached to the main body. The main body has two locking arms (246) each forming a slanted guiding portion (248) and a hook (249). Each fixing device has a top plate (42) for securing the fan thereon, and a support plate (46) for supporting the clips. A pair of tabs (49) and a pair of tongues (492) is formed at each of opposite ends of each fixing device, and defines a through hole (494) for receiving a corresponding locking arm. When the operating arm is rotated to raise the main body, the tongues guide the guiding portions and cause the hooks to tightly engage in locking holes (62) of a retention module (60).