Ultraviolet (UV) photodetectors (PDs) based on ZnO micro/nanowire (MNW) networks with Pt contacts have been fabricated on glass substrates. The PDs exhibited a high photosensitivity (5 ~ 103) for 365 nm UV light with a fast recovery time (0.2 s) at a reverse bias voltage of 2 V. The light induced modulation of Schottky barrier and MNW-MNW junction barrier was employed to account for the results. It was also observed that the PD had a high on-off ratio of 800 without external bias. The photovoltaic effect was proposed to explain the self-powered phenomenon.