To improve the performance of ZnO thin film transistors (TFTs) by using appropriate metal contacts, the different roles of contact materials between oxidation interlayer and doping effect are investigated. With careful characterization, an oxidation interlayer has been verified at the interface between ZnO film and Al or Ti contact, which is suggested to be responsible for contact resistance and thermal reliability. On the other hand, it is observed that the doping effect is the main reason for the Sn or Cu contact characteristics. The superior contact using Sn is due to an oxidation-free interface, donor doping effect, and a low barrier height. By using a metal contact with a high Gibbs free energy, the metal layer would hardly consume oxygen from channel layer during sputtering and easily form no oxidation interlayer. Thus, choosing a metal contact is important when fabricating high-performance metal-oxide TFTs for flat-panel displays.