The optoelectrical characteristics of the ZnO nanoparticles (NPs) annealed in vacuum or oxygen condition from $200^{circ}C$ to $600^{circ}C$ were examined. Increased on-off ratio (or, the ratio of photocurrent to dark current) was observed when they were annealed at $300^{circ}C$, $400^{circ}C$ and $500^{circ}C$ with the values enhanced about 4 orders compared to the as-prepared ZnO NPs in both annealing conditions, while the maximum efficiency was shown at the annealing temperature of $600^{circ}C$ for the ZnO NPs annealed in vacuum with the value of 29.8 mA/W and at the temperature of $500^{circ}C$ for those annealed in oxygen condition with the value of 40.3 mA/W. Photoresponse behavior of the ZnO NPs annealed in oxygen showed the sharp increase right after the ir exposure to the light followed by the slow decay and saturation during steady illumination, differing from the ZnO NPs annealed in vacuum which only exhibited the gradual increase. This difference occurred due to the curing effect of the oxygen vacancies. SEM images indicated no change in their morphologies with annealing, indicating the change in their internal structures by annealing, and most remarkably at $600^{circ}C$. As for their photoluminescence(PL) spectra, the decrease of the deep-level(DL) emission was observed when they were annealed in oxygen at $400^{circ}C$, and not at $200^{circ}C$ and $600^{circ}C$.