Abstract: In this paper, the reliability function and the mean time to failure (MTTF) function are calculated based on Markov model and reliability block diagram model for inverter with parallel or standby redundancy structures from the component level to the system level, respectively. The boundary condition determining which structure has high reliability is obtained. The influence of each parameter in the boundary condition is analyzed, including the ambient temperature, the success rate of fault-tolerant mechanism, current, and thermal impedance. The reliability function of the N-phase inverter with redundant structure is proposed, and the method of determining the number of parallel branches and standby branches in the highly reliable redundant structure is given. Taking a three-phase inverter as an example, the accuracy of the boundary condition function is verified by simulation and experiment.