Ultraviolet photoresistors based on ZnO thin films were fabricated on sapphire substrates with MgO buffer layer by plasma-assisted molecular beam epitaxy. An extremely large dark resistance up to 4 x 10(10) Omega was obtained and the dark/photo resistance ratio is up to 2.3 x 10(5) with a light intensity of 1.3 mW/cm(2) at 370 nm. The spectral response shows a large responsivity of more than 1 Omega W--1(-1) in the UV region. The photo-resistance depends linearly on the reciprocal of the optical power density for more than two orders of magnitude. The transient response property shows a decay time of 167 mu s and the relaxation mechanisms are also discussed. (C) 2011 Elsevier Ltd. All rights reserved.