A semiconductor device is provided. A transistor is formed on a substrate, and a metal silicide layer is formed on the surface of a gate conductor layer and a source/drain region. Next, a surface treatment process is performed to selectively form a protection layer on the surface of the metal silicide layer. Then, a spacer of the transistor is partially removed using the protection layer as a mask, so as to reduce the width of the spacer. Then, a stress layer is formed on the substrate.