Three kinds of metal-oxide-semiconductor high-electron-mobility transistors （MOS-HEMTs） with different thickness of Al2O3 dielectric layer were fabricated on the same wafer by atomic layer deposition. The measurement results of MOS capacitance，gate leakage current，and the output and transfer characteristics indicate that the control capability of the gate on two-dimentional electron gas will be reduced，while the gate leakage current will be decreased and the breakdown voltage will be increased with the increase of the Al2O3 dielectric layer thickness. Our analysis shows that the thinner the dielectric layer is，the greater the gate capacitance will be，which leads to greater negative shift of threshold voltage and much poorer insulating performance in restraining gate leakage current. Otherwise，with the increase of dielectric layer，higher gate voltage will be applied to obtain higher maximum saturation current density. Moreover，a thorough analysis of the transconductance and the capacity-voltage （C-V） characteristic shows that better passivation effect and insulating performance can be obtained with thicker dielectric layers.