A method of fabricating a semiconductor device including a diffused metal-oxide-semiconductor (DMOS) transistor, an n-type metal-oxide-semiconductor (NMOS) transistor, and a p-type metal-oxide-semiconductor (PMOS) transistor includes forming separation regions in a semiconductor substrate, forming a gate insulating film, forming a DMOS gate electrode on the gate insulating film, forming a first mask pattern on the semiconductor substrate, performing a first ion implantation process, forming a second mask pattern on the semiconductor substrate, performing a second ion implantation process, forming a third mask pattern on the semiconductor substrate and performing a third ion implantation process into the semiconductor substrate, and forming a fourth mask pattern on the semiconductor substrate and performing a fourth ion implantation process.