An ultraviolet (UV) photodetector based on ZnO-reduced graphene oxide (ZnO-rGO) composites have been successfully fabricated. A pure ZnO photodetector was also fabricated by similar method. In comparison with the pure ZnO UV photodetector, the ZnO-rGO photodetector exhibits a much larger photocurrent and a better light-to-dark-current-ratio. The mechanism of photocurrent enhancement was investigated using I-V characteristics, photoluminescence (PL) spectra, transmittance spectra and time-dependent photocurrent analysis. Results show that the photocurrent enhancement of the ultraviolet photodetector is due to the improvement of the carrier lifetime, because the carrier recombination of ZnO were reduced by rGO. It provides a potential way to fabricate high-response UV photodetectors.