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Polymer/nanoparticle (NP) composite films have attracted great attention due to their potential applications in electrical bistable devices. The bistable mechanism is usually attributed to electron trapping and detrapping between the NP trap center and the polymer matrix. However, the exact conduction switching mechanism is still in controversy and even in the same polymer/NP system based devices the switch-on mechanism has been explained by two different models: either Fowler–Nordheim (FN) tunneling or trapped charge-limit-current (TCLC). Therefore, the study on the conduction switching mechanism for polymer/NP composite electrical bistable devices is critically necessary. In this work, ZnO NPs embedded in poly (ethylene oxide) (PEO) were first applied in electrical bistable devices using a solution process and the effect of the nanoparticle surface defects on the conduction switching mechanism is studied. The electrical bistability is observed from the device with the structure ITO/PEO : ZnO-NPs/Al and the conduction switch-on process is dependent on the existence of ZnO surface defects. The effect of the nanoparticle surface defects was investigated by current–voltage, Scanning Electron Microscopy (SEM), and photoluminescence measurements. Besides effectively separating nanoparticles, the surface capping can passivate the surface defects and affect the electrical hysteresis. The switch-on mechanism for the devices based on the NPs with surface detects can be modeled by TCLC while the one based on the NPs with the complete surface defect passivation can be explained by FN tunneling. The results demonstrate that the FN tunneling induced conduction switch-on process is more desirable in electrical bistable devices due to the better device performances.