In this letter, we present high-performance AlGaN/GaN high electron mobility transistor (HEMT) by 3-dimensional (3D) tri-gate. Due to the excellent channel control of tri-gate structure, the tri-gate HEMT shows the low subthreshold swing (SS) of 65 mV/decade, low off-state drain leakage current (Ioff) of 7 nA/mm, and On/Off ratio up to 7.6 × 107. Furthermore, we combined the tri-gate structure and metal oxide semiconductor (MOS) structure. The tri-gate MOS-HEMT with high-k dielectric oxide HfO2 was fabricated. The good interface between HfO2 and AlGaN and strain-induce polarization increase the carrier concentration. Compared to the tri-gate HEMT, the increased maximum drain current, higher On/Off ratio and better breakdown voltage is demonstrated by implanted the high k dielectric HfO2. The improved electrical performance of tri-gate MOS-HEMT shows potential for next generation power application.