The capacitance-voltage (C-V) characteristic of InSb metal-oxide-semiconductor (MOS) device was measured at 100 K. In the inversion region, the second capacitance plateau was observed for the first time, which can be attributed to the electron filling in the second subband. At the same time, a resonant defect state was found for the first time. By using the previously presented C - V model in the non-quantum limit condition, the subband structure in the inversion layer has been obtained.