A method of fabricating a semiconductor device includes forming an active region in a semiconductor substrate, forming a plurality of dummy gates on the active region, the plurality of dummy gates having a gate mask disposed thereon, forming an interlayer insulating layer on the gate mask, and performing a one-time chemical mechanical polishing (CMP) process by using a slurry composition capable of polishing the interlayer insulating layer and the gate mask until top surfaces of the dummy gates are exposed.