We have fabricated buried channel MOSFETs with a thermally grown gate oxide in 4H-SiC. The gate oxide was prepared by dry oxidation with H₂O annealing. The buried channel region was formed by nitrogen ion implantation at room temperature followed by annealing at 1500 ^oC. The optimum doping depth of the buried channel region has been investigated. For the nitrogen concentration of 1x10¹⁷ cm^-3, the optimum depth was found to be 0.2um. Under this condition, the channel mobility of 140 cm²/Vs was achieved with the threshold voltage of 0.3 V. This channel mobility is the highest reported so far for a enhancement type 4H-SiC MOSFET with a thermally grown gate oxide.