The effect of interface state charges on the field-effect mobility of n-channel 6H-SiC MOSFET is analyzed based on the nonuniform distribution of interface state density in the energy gap. The results of the analysis show that interface state charges have the influence of lowering the field-effect mobility in n-channel SiC MOSFET. A relationship has been established between the ratio of the experimentally determined field-effect mobility to the inversion-layer carrier mobility and interface states.