The experiment result shows that the holes generated by avalanche can be injected into gate oxide of a NMOSFET biased into snapback, and then both hole trapping and interface state generation can be found. These trapped holes may recombine with electrons tunneling into gate oxide due to the ultrathin gate oxide, and then many neutral electron traps would be generated and the gate oxide current would be increased. The threshold voltage would increase and the sub-threshold current would decrease when the injected electrons are trapped by the electron traps. The degradation of drain leakage current can be divided into two phases. Sub-threshold current is predominant in the first phase, while in the second phase gate current is predominant. After pre-HE stress, the generated interface states can reduce the number of holes being injected into gate oxide generated by avalanche process during snapback stress, which causes the MOSFET snapback degradation to decrease in on-state and off-state modes.