Unipolar resistive switching behaviors including bistable memory switching and monostable threshold switching were found in ZrO2 thin films fabricated by a simple sol-gel method with the Ti/ZrO2/Pt structure. The multilevel resistive switching behaviors were also revealed by varying the compliance current from 9 to 38 mA. Physical mechanisms based on a conductive filament model were proposed to explain the resistive switching phenomena and the device breakdown. A figure of merit Z = ρa=ρf was defined as a criterion for evaluating OFF/ON resistance ratio where ρf and ρa represent the resistivities of the conductive filament and the fracture region of the filament, respectively. The advantages such as unipolar resistive switching, multilevel resistive switching, good scalability, low operation voltage (<5 V), high OFF/ON resistance ratio (>103) nondestructive readout, long retention (>104 s), and simple fabrication method make the ZrO2-based resistive switching device a promising candidate for next-generation nonvolatile memory applications.