We report resistive switching characteristics in Pt/ZnO/Pt devices where ZnO thin film is fabricated at various oxygen conditions. With the increase of oxygen contents in ZnO thin film, the forming voltage is gradually increased while reset and set voltages are almost unchanged. We also investigated the effect of top electrodes on resistive switching of top electrode/ZnO/Pt device. For a Pt/ZnO/Pt device, it exhibits the excellent resistive switching behavior due to high electrical field of the well-defined Schottky barrier. For Al/ZnO/Pt device, little resistive switching phenomena were occurred due to leakage current of the weak Schottky (or Ohmic) contact.