This paper investigates the temperature-dependent performances of AlGaN/GaN metal-oxide-semiconductor high electron mobility transistor (MOS-HEMT). The gate dielectric layer and surface passivation layer are formed by the H2O2 oxidation technique. The gate dielectric quality is estimated by the breakdown electric field \((E_{{{\rm BD}}})\) and low-frequency noise. The capacitance-voltage \((C-V)\) hysteresis characteristics of MOS and Schottky diodes at 300/480 K are also studied. An appropriate thermal model is used to investigate the self-heating effect and calculate the effective channel temperature \((T_{{{\rm eff}}})\) . The dc performances of the present MOS-HEMT are improved at 300/480 K, as compared with a Schottky-barrier HEMT (SB-HEMT), including output current density, maximum extrinsic transconductance \((g_{{m,{\max }}})\) , gate voltage swing, gate-drain leakage current \((I_{{{\rm GD}}})\) , specific ON-resistance \((R_{\mathrm{{\scriptstyle ON}}})\) , three-terminal OFF-state breakdown voltage \(({\rm BV}_{\mathrm{{\scriptstyle OFF}}})\) , and subthreshold swing. Factors that cause \(I_{{{\rm GD}}}\) and \({\rm BV}_{\rm {{OFF}}}\) are analyzed by the temperature-dependent measurement. The passivation effect of the present MOS-HEMT is also confirmed by the surface leakage measurement. The devised MOS-HEMT demonstrates superior thermal stability to the reference SB-HEMT. The present-design is promising for high-temperature electronic applications.