We carry out reliablity tests and investigate the failure mechanisms for wafer level vacuum packaged (WLVP) MEMS resonator using an accelerated degradation test (ADT). ADT is also used to estimate the mean life-time (or failure) of WLVP. The main failure mechanism of WLVP is found to be outgassing inside the package. Failure distribution of log-logistic is well fitted with the failure data from ADT, and Arrhenius model is used as a stress-life model because the main stress parameter is temperature. Acceleration factor (AF) for temperature levels is calculated and used to evaluate mean life-time of the WLVP after redesign. Finally the accuracy of ADT model is examined by verification test. The successful WLVP is achieved by reducing outgassing through the deposition of titanium coating inside the package as a getter material.