The dependence of effective channel mobility (mu(eff)) in inversion layer of 4H-SiC MOSFETs on (11 (2) over bar0) face with a wide range of substrate impurity concentrations (10(15) to 10(17) cm(-3)) on the effective normal field (E-eff) has been investigated. The maximum value of the mu(eff) decreases with the increase in the substrate impurity concentration while the mu(eff) tends to converge into a unique curve in higher E-eff region independent of the substrate impurity concentration. The measurement temperature dependence of the mu(eff) indicates that the mu(eff) is predominately governed by phonon scattering mechanism. It is also shown that the mu(eff) is improved due to the gate oxidation at lower temperatures, suggesting the affect of the interface states at SiO2/SiC interface.