Example embodiments provide a complementary metal-oxide semiconductor (CMOS) semiconductor device and a method of fabricating the CMOS semiconductor device. The CMOS semiconductor device may include gates in the nMOS and pMOS areas, polycrystalline silicon (poly-Si) capping layers, metal nitride layers underneath the poly-Si capping layers, and a gate insulating layer underneath the gate. The metal nitride layers of the nMOS and pMOS areas may be formed of the same type of material and may have different work functions. Since a metal gate is formed of identical types of metal nitride layers, a process may be simplified, yield may be increased, and a higher-performance CMOS semiconductor device may be obtained.