A fixing device for securing a heat sink to a CPU module includes a base plate, two elastic side plates respectively extending from two distal ends of the base plate, two curved portions formed between the base plate and the two elastic side plates for increasing elasticity of the fixing device when the two elastic side plates are expanded outward with respect to a central axis therebetween, two hooks respectively formed at free ends of the elastic side plates for securing to a backplane of the CPU module thereby sandwiching the heat sink between the fixing device and the CPU module, and two is engagement tabs respectively formed on the elastic side plates for allowance of an external tool to release the fixing device from the CPU module.