A method of fabricating a semiconductor device is disclosed. The method of fabricating a semiconductor device provides a semiconductor substrate; forming a gate stack overlying the semiconductor substrate; forming spacers each having a first inner spacer and a second outer spacer on sidewalls of the gate stack; forming a protective layer on sidewalls of the spacers, covering a part of the semiconductor substrate, wherein an etching selectivity of the protective layer is higher than that of the first inner spacer.