The cause of the device degradation in a SONOS structure has been investigated with using two different gate dielectric structures of the stacked oxide-nitride-oxide (ONO) and conventional single-layered oxide. It is found that the device degradation behaviors for the stacked-ONO gate structure have different polarity dependence from the conventional single oxide gate, especially under positive F-N gate stress condition. It is shown that the bi-modal device degradation behaviors can be explained by the fluence of high energetic anode hole based on the anode hole injection (AHI) model.