In this study, a NiO/diamond UV-photodetector has been fabricated and investigated. A single crystal diamond (SCD) layer was grown on a high-pressure-high-temperature Ib-type diamond substrate by using a microwave plasma chemical vapor deposition system. NiO films were deposited directly by the reactive magnetron sputtering technique in a mixture gas of oxygen and argon onto the SCD layer. Gold films were patterned on NiO films as electrodes to form the metal-semiconductor-metal UV-photodetector which shows good repeatability and a 2 orders of magnitude UV/visible rejection ratio. Also, the NiO/diamond photodetector has a higher responsivity and a wider response range in contrast to a diamond photodetector.