MEMS (microelectromechanical system) reliability has been a very important issue, especially for safety-critical applications. Due to the diversity and multiple energy domains involved, MEMS devices are vulnerable to various failure mechanisms. MEMS reliability under different failure mechanisms should be analyzed separately. Since most of MEMS devices contain movable parts, material fatigue and aging under long-term repeated cycling load may lead to potential device failure, which in turn degrades the device reliability. In this paper, the reliability of poly-silicon MEMS comb accelerometers under material fatigue failure mechanism is analyzed. Based on ANSYS stress simulation, the mean-time-to-failure (MTTF) lifetimes and failure rates for both BISR (built-in self-repairable) and non-BISR poly-silicon MEMS comb accelerometers are derived. Simulation results show that the fatigue lifetime of MEMS accelerometers made by poly-silicon material can be good enough for general purpose applications. However, for some "weak" devices with certain structure defects, the material fatigue and aging may become potential threats. Compared to non-BISR design, BISR MEMS accelerometer demonstrates effective reliability improvement due to redundancy repair. MEMS reliability under material fatigue for other MEMS materials will be further studied in the future.