A method for forming a metal oxide semiconductor (MOS) transistor is provided. First, a gate structure is formed over a substrate. Then, offset spacers are formed on respective sidewalls of the gate structure. A first ion implantation process is performed to form a lightly doped drain (LDD) in the substrate beside the gate structure. Other spacers are formed on respective sidewalls of the offset spacers. Thereafter, a second ion implantation process is performed to form source/drain region in the substrate beside the spacers. Then, a metal silicide layer is formed on the surface of the source and the drain. An oxide layer is formed on the surface of the metal silicide layer. The spacers are removed and an etching stop layer is formed on the substrate. With the oxide layer over the metal silicide layer, the solvent for removing the spacers is prevented from damaging the metal silicide layer.