The bipolar resistive switching characteristics of a Pt/ZnO/Pt structure were investigated in this study. It is found that low resistance state can be only formed at a positive bias while high resistance state can be only formed at a negative bias with the bipolar switch characteristics. The conduction mechanism at different resistance states also were discussed. The resistive switch mechanism may be relate to the generation and oxidation of filaments and the switch polarity is likely resulted from the geometric effect due to the asymmetrical structure of electrodes, resulting in the flux of defects near the edge of top electrode caused by local enhancement of electric field. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3603989] All rights reserved.