A replacement gate process is disclosed. A substrate and a dummy gate structure formed on the substrate is provided, wherein the dummy gate structure comprises a dummy layer on the substrate, a hard mask layer on the dummy layer, spacers at two sides of the dummy layer and the hard mask layer, and a contact etch stop layer (CESL) covering the substrate, the spacers and the hard mask layer. The spacers and the CESL are made of the same material. Then, a top portion of the CESL is removed to expose the hard mask layer. Next, the hard mask layer is removed. Afterward, the dummy layer is removed to form a trench.