A new model of oxidation is proposed to describe the growth of both thin and thick silicon dioxide layers. The assumption of the model is the presence of an exponential distribution of total net charges during oxidation. In this paper, we take into account the influence of the oxide charges and derive a new thermal-oxidation relationship of silicon, which agrees reasonably well with experimental data covering both thin and thick oxides. The relationship can be reduced to the well-known formula of Deal and Grove in the limit of thick oxide. With this model, we can also explain the effect of an external field on the oxidation rates satisfactorily.