Diamond is a valuable material with unique properties of nanocrystalline and is widely used in the fabrication of nano-electronic devices to develop new and promising power device applications. In general, the hydrogenated-(C-H) nano-diamond Metal Oxide Semiconductor Field Effect Transistor (MOSFET) depicts the normally-on status (depletion mode). In this paper, we investigate the interface charge effect on C-H diamond to confirm the normally-on operation, then show the characterization of surface charge effect on device operations, including normally-off with a controlled gate voltage and the nature charge of the power device. To study the corresponding effects, we simulate the two-dimensional (2D) C-H nano-diamond MOSFET under several surface charge models. These negatively charged sites or acceptors are scattering centers for carrier (holes) transport near the C-H surface. When FET operation can be realized without negatively charged sites, channel mobility is not limited by these sites and is enhanced by a factor of 3-4. The results confirm that the two-dimensional hole gas (2DHG) close to the surface indicates a p-type channel due to the dipole effect between hydrogenated diamond and the negative charge of Al2O3. The normally-off operation is achieved to realize a safety point for the power device. The evaluation results also show that the threshold voltage shifts to a negative value in a positive charge model, given that in principle, this state is not feasible without an oxidation layer or doping.