The reaction processes at the interface between SiC with 4H structure (4H-SiC) and SiO 2 during wet oxidation are investigated by electronic structure calculations within the density functional theory. Our calculations for 4H-SiC/SiO 2 interfaces with various orientations demonstrate characteristic features of the reaction depending on the crystal orientation of SiC: On the Si-face, the H 2 O molecule is stable in SiO 2 and hardly reacts with the SiC substrate, while the O atom of H 2 O can form Si-O bonds at the C-face interface. Two OH groups are found to be at least necessary for forming new Si-O bonds at the Si-face interface, indicating that the oxidation rate on the Si-face is very low compared with that on the C-face. On the other hand, both the H 2 O molecule and the OH group are incorporated into the C-face interface, and the energy barrier for OH is similar to that for H 2 O. By comparing the calculated energy barriers for these reactants with the activation energies of oxide growth rate, we suggest the orientation-dependent rate-limiting processes during wet SiC oxidation.