An enhancement-mode (E-mode) AlGaN/GaN metal–insulator–semiconductor high-electron-mobility transistor (MIS-HEMT) with La2O3/SiO2 gate insulator is investigated for high power application. The La2O3/SiO2 composite oxide formed amorphous La-silicate after post deposition annealing. Good oxide film quality and excellent La-silicate/AlGaN interface properties were achieved as evidenced by the capacitance–voltage (C–V) curves and hysteresis effect of the La-silicate on AlGaN/GaN metal–oxide–semiconductor capacitors. As a result, the E-mode AlGaN/GaN MIS-HEMT with La-silicate gate insulator shows good threshold voltage (Vth) stability and demonstrated only slightly increase in the dynamic on-resistance (Ron) after high drain bias stress test. The device also exhibits high current density of 752 mA/mm, high maximum transconductance of 210 mS/mm, low subthreshold swing of 104 mV/decade, and ION/IOFF = 107 when tested at VDS = 10 V. Furthermore, low on-resistance of 7.6 Ω mm, high breakdown voltage of 670 V, and excellent delay time of 4.2 ps were achieved, demonstrating the La-silicate MIS-HEMTs have the potential to be used for power electronic applications.