Zinc oxide (ZnO) nanoparticles were synthesized from microemulsions in a microchannel reactor system. The microemulsions provide confined space for the reactants, which is favorable for controllable reaction and nucleation, thus avoiding the formation of large particles. In addition, the microemulsions prevent the deposition of ZnO particles on the wall of the microchannels of the reactor. Three Zn2+ sources (Zn(NO3)(2), ZnSO4, and ZnCl2) were tested in the synthesis of ZnO nanoparticles. Among them, Zn(NO3)(2) showed best performance, yielding ZnO particles with the smallest average grain size. The effects of Zn2+ concentration, reaction temperature, and feed flow rate on the average particle size of ZnO nanoparticles were investigated. At optimal conditions, ZnO nanoparticles with average size of 16 nm were obtained. The synthesized ZnO nanoparticles were characterized by scanning electron microscope (SEM), X-ray diffraction (XRD), UV-vis absorption spectroscopy, and a laser particle size analyzer. (C) 2013 Elsevier B.V. All rights reserved.