A series of simulations were carried out to investigate the failure mechanism of large area silicon power diodes. Static breakdown behavior of high voltage diodes is first investigated at elevated temperatures. Sustain-mode dynamic avalanche is then discussed. It was found that under isothermal and homogeneity condition, the reverse bias safe operation area (RBSOA) of power diodes is the same as the sustain-mode dynamic avalanche. After introducing current inhomogeneity, the failure will occur at reverse power density that is much less than the RBSOA predicted by the sustain-mode dynamic avalanche. The failure mechanism of power diodes is then proposed based on these findings.