A method of fabricating a semiconductor device includes forming a first gate pattern and a dummy gate pattern on a first active area and a second active area of a substrate, respectively, the first gate pattern including a first gate insulating layer and a silicon gate electrode, removing the dummy gate pattern to expose a surface of the substrate in the second active area, forming a second gate pattern including a second gate insulating layer and a metal gate electrode on the exposed surface of the substrate, the first gate insulating layer having a thickness larger than a thickness of the second gate insulating layer, and forming a gate silicide on the silicon gate electrode after forming the second gate pattern.