Provided is a metal-oxide semiconductor (MOS) transistor containing a metal gate pattern. The semiconductor device includes a p-channel metal-oxide semiconductor (PMOS) transistor including a semiconductor substrate, a first insulating film formed on the semiconductor substrate, a first metal gate conductive film formed on the first insulating film, and a nitrogen diffusion blocking film formed between the first insulating film and the first metal gate conductive film, and an n-channel metal-oxide semiconductor (NMOS) transistor including the semiconductor substrate, a second insulating film formed on the semiconductor substrate, a second metal gate conductive film formed on the second insulating film, and a reaction blocking film formed of metal nitride and formed between the second insulating film and the second metal gate conductive film. According to the inventive concept, a reaction between a metal gate film and an insulating film may be minimized so as to result in a highly reliable MOS transistor.