Reliability assessment of SiN MIM capacitors in GaN MMIC is performed by constant voltage stress test. Two kinds of failure modes, critical charge density at which the dielectric breaks down and mean time prior to failure are investigated. The trap energy level in SiN dielectric is obtained by temperature dependent current characteristics. The degradation mechanism of SiN MIM capacitor is analyzed. The research shows that new donor-like traps are generated at dominant position during the stress. And the trap energy level becomes deeper after stress. The increased trap accelerates the scattering of the carrier, which leads to the decrease of leakage current in the end. The investigation on the failure mechanism of SiN MIM capacitor provides a reference for reinforcing the dielectric capacitors.