In this paper, we describe highly reliable GaN high electron mobility transistors (HEMTs) for high-power and high-efficiency amplifiers. First, we present the reliability mechanisms and progress on the previously reported GaN HEMTs. Next, we introduce our specific device structure of GaN HEMTs for improving reliability. An n-GaN cap and optimized buffer layer were used to suppress the trap-related phenomena, such as a current collapse. Gate edge oxidation is effective for reducing the gate leakage current. A Ta-based barrier metal was inserted between an ohmic electrode and interconnection metal for preventing increase in contact resistance. SiN of passivation film was optimized for reducing the current collapse of short-gatelength HEMTs.