Reproducible resistance switching phenomenon in metal oxides is attributed to the non-linear oxygen ions transport. Here we present a phenomenological model to provide a unified explanation for both the unipolar and bipolar resistive switching mechanism. Numerical simulation results reveal the switching mode is determined by the electrode/oxide interface property. Without/with an interfacial barrier, unipolar/bipolar switching behavior is obtained. Also, the voltage-time dilemma between fast switching and long retention is explained by the non-linearity of the ionic transport under high electric field. Experimental data are employed for model verification.