Thermal etching was investigated as one of the ways to obtain an atomically flat surface of SiC substrates. The formation of graphite layer on substrate surface was suppressed by reducing the C/Si ratio in the vapor phase. Though (11 (2) over bar0) substrates that etched in argon atmosphere bad round shaped pits on the surface, such pits were not observed in nitrogen atmosphere. Atomically flat substrate surface with the RMS roughness of approximately 0.3 nm was obtained in nitrogen atmosphere even with the high etching rate of 250 mum/h. The substrates with higher nitrogen concentrations had higher surface flatness and higher etching rate. Thermal etching method turned out to be an effective technique to obtain the substrates that suit for the crystal growth-and/or the device fabrication. The difference of etching mechanism between (0001) surface and (11 (2) over bar0) surface was compared, and it was discussed with a consideration of bond configuration on the surface.