V-ramp method was used to evaluate gate oxide reliability of 0.18μm dual gate CMOS process. Charge of breakdown (Qbd) and voltage of breakdown (Vbd) of gate oxide with n-type substrate and p-type substrate were extracted. It was found that for low voltage (thin oxide) gate oxide device the Qbd of gate oxide of n-type substrate and p-type substrate are almost the same, but for high voltage (thick oxide) gate oxide device the Qbd of n-type substrate and p-type substrate have a big difference. At the same time, Qbd of gate oxide with p-substrate is bigger than that of gate oxide with n-substrate. The difference of Qbd of thin gate oxide and thick gate oxide can be attributed to lithographic damage to the interface of poly-silicon gate and thick gate oxide. There is a big difference between the Weibull slopes of charge of breakdown of thin oxide and thick oxide. For the voltage of breakdown, Similar difference between n-substrate and p-substrate gate oxide was also observed. However, there is no big difference between the Weibull slopes of voltage of breakdown of thin oxide and thick oxide.