The invention discloses a method for fabricating a MOS transistor. A substrate having thereon a gate structure is provided. A silicon nitride layer is deposited on the gate structure. A dry etching process is then performed to define a silicon nitride spacer on each sidewall of the gate structure and a recess in a source/drain region on each side of the gate structure. A transitional layer covering the gate structure and the recess is deposited. A pre-epitaxial clean process is performed to remove the transitional layer. The substrate is subjected to a pre-bake process. An epitaxial growth process is performed to grow an embedded SiGe layer in the recess. The disposable silicon nitride spacer is removed.