The interface trap density near the conduction band in SiO2/SiC interface was evaluated by making capacitance-voltage measurements for gate-controlled diodes using the metal-oxide-semiconductor field effect transistors (MOSFETs). The close correlation between channel mobility and the shallow trap density was clearly found for the 4H- and 6H-SiC devices prepared with various gate-oxidation procedures. This result is strong evidence that a significant cause of the poor inversion channel mobility of SiC MOSFETs is the high density of shallow traps between the conduction band edge and the surface Fermi level at the threshold voltage.