Abstract: AlGaN/GaN high electron mobility transistors (HEMTs) have excellent physical and chemical stability, which gives it great potential for use in consumer, industrial and space applications. However, the defects of epitaxial growth AlGaN/GaN HEMTs are difficult to completely remove, and high-density defects strongly affect the radiation resistance of the devices. Therefore, understanding the evolution and mechanism of defects under particle irradiation is of great significance for improving the radiation resistance. The degradation mechanisms in AlGaN/GaN HEMTs irradiated by 3 MeV protons and 14 MeV neutrons were investigated. After 3 MeV proton irradiations with a flux of 4×10¹⁴ cm^-2 and 1×10¹⁵ cm^-2 and 14 MeV neutron irradiations with a flux of 1.2×10¹² cm^-2 and 2×10¹³ cm^-2, the AlGaN/GaN HEMTs shows decrease in saturation drain current and peak transconductance, positive drift in threshold voltage. The defects of AlGaN/GaN HEMTs induced by irradiation were tested and studied by deep level transient spectroscopy (DLTS). The decreasing of defect concentration induced by 3 MeV proton irradiation reduces the reverse gate leakage current, while 14 MeV neutron irradiation causes defect concentration increases, increasing the reverse gate leakage current. According to the defect energy levels after proton and neutron irradiation are both (0.850±0.020) eV, it is inferred that the defect types are nitrogen interstitial defects. The displacement damage effect caused by the displacement of nitrogen interstitial defects after neutron and proton irradiation is the main reason for the electrical performance degradation of AlGaN/GaN HEMT devices.