In addition to their sizes and morphologies, the electronic and optical properties of ZnO nanostructures are strongly dependent on the nature and concentration of point defects generated during the crystallization process. Herein, we report synthesis of ZnO nanoparticles with high defect concentration through fast crystallization under microwave irradiation. ZnO nanocrystallites, obtained through microwave assisted hydrothermal synthesis process, exhibited higher visible light photocatalytic activity than the conventionally hydrolyzed nanoparticles as well as manganese (Mn) doped nanoparticles (ZnO:Mn(2+)). The enhancement in the photocatalytic activity was attributed to the higher defect concentration in the as-synthesized nanocrystallites which was confirmed from their optical absorption and photoluminescence spectra. Microwave assisted hydrolysis could be an effective and convenient alternate method to introduce defects in ZnO nanocrystallites which enhances visible light photocatalytic activity of ZnO nanocrystals. (C) 2011 Elsevier B.V. All rights reserved.