The silicon dioxide film of thickness 150? with device quality has been nitrided thermally by conventional long time method and high temperature rapid method. For the films breakdown characteristics and endurance under high field were investigated. These results indicate that after nitridation, the distribution of breakdown field strength narrows, its dependence on electrode area decreases, and the maximum breakdown field strength degrades slightly. At the same time, SiO2/Si interface stability under high field and time-dependent dielectric breakdown characteristics are improved by thermal nitridation. The improvement depends strongly oh both the electrode direction of applied gate voltage and nitridation processing condition. According to the mechanism of current transport, we suggest a breakdown model in which charge accumulation, trapping density and position of its center of gravity are all considered.