The efficiency of visible-light-driven oxide semiconductor photocatalysts for hydrogen production from water splitting has long been restricted by the large particle size. In this study, a simple and versatile microwave-assisted synthesis of InVO4 nanocrystal photocatalysts is proposed. The as-prepared products possess high crystallinity and ultra-small size (20 nm) that has never been reported yet. The 20 nm-sized nanocrystal samples show reduced visible-light response and enlarged reductive energy compared with large-sized InVO4 samples. The hydrogen production rate of InVO4 nanocrystal photocatalysts is 14.16 mu mol h(-1) g(-1), which is higher than that of large-sized InVO4 microspheres (9 mu mol h(-1) g(-1)). The model of size-dependent activities and the possible mechanism for the activity enhancement are also discussed by the band position calculation in this paper. (C) 2013 Elsevier B.V. All rights reserved.