We have investigated the oxidation and post oxidation annealing (POA) effects on the capacitance-voltage (C-V) characteristics and the interface state density (D(it)) of n-type SiC MOS structures formed on the C(000 (1) over bar) face. In addition, we analyzed the SiC surface and the SiO(2)/4H-SiC interface by atomic force microscopy (AFM) and transmission electron microscopy (TEM). Oxidation and a subsequent high-temperature hydrogen annealing reduced Bit near the conduction-band edge. However, the value of D(it) at the energy level from the conduction band edge (E(c)-E) approximate to0.6eV is one order of magnitude higher than that of n-type MOS structures formed on the Si(0001) face, Moreover, AFM revealed that the surface roughness of C(000 (1) over bar) face was comparable with that of Si(0001) face. TEM presented that the SiO(2)/SiC interface on the 4H-SiC C(0001) face was rougher than that on the 4H-SiC Si(0001) face.