The failure mechanism of drain bias TDDB for HV De-PMOS was studied, which is induced by minority, electrons for PMOS, from ion impact ionization at high V d injection to drift region dielectrics. Multistage Ig-t curve behavior was observed during off-sate drain bias TDDB test, electrons injected into drift region (Ig increase) at initial stage, electrons trapped in drift region dielectrics (Ig decrease) at 2 nd stage and percolation path formed (gate oxide breakdown) at last stage. Furthermore, power law model, used to extrapolated drain bias TDDB lifetime, and its characteristics, n value, Ea, width factor were studied. Finally, through quick method to screen effective result from various optimized process experiments, drain bias TDDB lifetime was significantly improved.