The degradation of thin tunnel gate oxide under constant Fowler-Nordheim current stress was studied using capacitors.The degradation is a function of constant current and time，which depends more on the magnitude of constant current.Thus the degradation is a strong function of injected charge density Qinj. Positive charge trapping is usually dominant at lower Qinj followed by negative charge trapping at higher Qinj，causing a reversal of gate voltage change.