Highly c-axis-oriented ZnO films were successfully deposited on the nucleation sides of freestanding diamond films by RF reactive magnetron sputtering. I-V characteristics of ultraviolet (UV) photodetectors with ZnO/diamond structure were studied and a significant photoresponse was observed under UV light illumination. The dark-current and the photocurrent of the ZnO photodetectors were relative to the grain size and the quality of ZnO films. For the photodetector with a bigger grain size, a weaker dark current and a stronger photocurrent were obtained under 10 V bias voltage. The photocurrent rise and decay process confirmed the carrier-trapping effect. (c) 2007 Elsevier B.V. All rights reserved.