Breakdown characteristics of the thin gate oxide are measured under constant voltage stresses. Breakdown mechanism of time-dependent dielectric breakdown are studied and effects of the areas of the gate oxide on breakdown characteristics are discussed. Breakdown charge QBD is measured and analyzed, the results show that breakdown charge QBD is not constant, it depends on the areas of the gate oxide and the voltage of the gate. Relative coefficients are fitted and analytical expression of QBD is presented in the paper.