A method for manufacturing a semiconductor device is provided. A first stack structure and a second stack structure are formed to respectively cover a portion of a first fin structure and a second fin structure. Subsequently, a spacer is respectively formed on the sidewalls of the fin structures through an atomic layer deposition process and the composition of the spacers includes silicon carbon nitride. Afterwards, a interlayer dielectric is formed and etched so as to expose the hard mask layers. A mask layer is formed to cover the second stack structure and a portion of the dielectric layer. Later, the hard mask layer in the first stack structure is removed under the coverage of the mask layer. Then, a dummy layer in the first stack structure is replaced with a conductive layer.