SiN-passivated AlGaN/GaN high electron mobility transistors (HEMTs) are exposed to 1 MeV neutron at fluences up to 1015 cm-2. The device shows a negligible degradation at neutron fluences below 1014 cm-2, while the gate leakage current (Ig) slightly changes (the forward IF increases, the reverse IR decreases.) at low fluencies and the IR degrades dramatically at fluences higher than 1014 cm-2. Moreover, near the knee voltage, the transconductance decreases at fluences up to 1015 cm-2, but the Schottky characteristicis become degraded after neutron irradiation. And the 20-hour annealing results do not show any significant annealing recovery effect at room temperature, while the parameters also continues to degrade a little. Therefore, the drain current (near the knee voltage) and the IF degradation of SiN-passivated AlGaN/GaN HEMT can be attributed to the irradiation induced defects in SiN passivation layers, demonstrating that the effectiveness of the SiN layer in passivating surface state in the source-gate spacer and gate-drain spacer is undiminished by neutron irradiation. And the Ohmic contact is so relatively robust to neutron, but the Schottky characteristics degrade obviously. The annealing results prove that the damage induced by neutron may be recovered more difficultly. SiN-passivated AlGaN/GaN HEMT appear to be an attractive candidate for space and terrestrial applications where resistance to displacement damage is required.