A method of forming a metal-oxide-semiconductor (MOS) transistor device is disclosed. A semiconductor substrate is prepared first, and the semiconductor substrate has a gate structure, a source region and a drain region. Subsequently, a stress buffer layer is formed on the semiconductor substrate, and covers the gate structure, the source region and the drain region. Thereafter, a stressed cap layer is formed on the stress buffer layer, and a tensile stress value of the stressed cap layer is higher than a tensile stress value of the stress buffer layer. Since the stress buffer layer can prevent the stressed cap layer from breaking, the MOS transistor device can be covered by a stressed cap layer having an extremely high tensile stress value in the present invention.