In this work. ZnO nanoparticles were prepared using ZnSO4 and NH4HCO3 aqueous solutions as feedstocks in a membrane dispersion micro-structured reactor exhibiting efficient mixing. The effects of calcination temperatures for the precursors, calcination time, feed concentrations, and feed flow rates on the crystal sizes of ZnO nanoparticles were investigated. The ZnO nanoparticles obtained were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), and Brunauer-Emmet-Teller analysis (BET). The XRD results indicate that the prepared ZnO nanoparticles had pure hexagonal structures with an average crystal size of about 9.4 nm. The specific surface area could reach 74.2 m(2)/g; meanwhile, the TEM photographs show that the ZnO nanoparticles were of a pseudo-spherical shape with an average particle size of about 9.33 nm, which was very close to the XRD results. Additionally, the calcination temperatures were found to have a great effect on diameters and purities of ZnO nanoparticles. Advantages of this method include the small and narrow particle size distribution of prepared ZnO nanoparticles, the cheap raw materials, and the continuous operation mode. (C) 2010 Elsevier B.V. All rights reserved.