A CMOS transistor and a method for manufacturing the same are disclosed. A semiconductor substrate having at least a PMOS transistor and an NMOS transistor is provided. The source/drain of the PMOS transistor comprises SiGe epitaxial layer. A carbon implantation process is performed to form a carbon-doped layer in the top portion of the source/drain of the PMOS transistor. A silicide layer is formed on the source/drain. A CESL is formed on the PMOS transistor and the NMOS transistor. The formation of the carbon-doped layer is capable of preventing Ge out-diffusion.