ZnWO4 nanorod photocatalysts with various aspect ratios were synthesized by a hydrothermal method. The aspect ratio of the ZnWO4 nanorods was governed by the hydrothermal temperature and pH value. ZnWO4 nanorod photocatalysts showed highly efficient photocatalytic activity for the degradation of methylene blue under ultraviolet light irradiation. The photocatalytic activity and the effective separation of photogenerated electron-hole pairs were promoted by the increasing aspect ratio of the ZnWO4 nanorods. The amount of electron donors in the ZnWO4 increased with the aspect ratio of ZnWO4, which was in good agreement with photocurrent results. The enhancement of photocurrent density and photocatalytic activity resulted from the reduction of charge-transfer resistance and capacitive reactance, which were deduced by the high aspect ratio of ZnWO4 nanorods.