Resistive random access memory (RRAM) is considered to be one of the important candidates for the next generation of memory devices. Zinc oxide resistive memory has also been studied for many years, but there are still some controversial topics and problems. Herein, an unusual resistance state has been observed in devices following the measurement and analysis of ZnO resistive memories with different thicknesses, a middle resistance state was speculated to explain the instability of ZnO RRAM. According to this speculation, a two-layer structure ZnO RRAM has been designed to significantly increase the device performance with the introduction of an HfO2 layer and the enhancement has also been explained based on the results of first-principles calculations.