Zinc oxide nanoparticles have been successfully prepared by a facile route involving the reaction of zinc sulphate heptahydrate and sodium hydroxide through drop-by-drop mixing synthesis-IA, instant mixing synthesis-IA and under the influence of microwave radiations. The synthesis under different reaction conditions played an important role and led to the formation of zinc oxide nanoparticles of different size and shapes. The synthesized nanoparticles were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques. The concentration dependent antimicrobial activity of synthesized ZnO nanoparticles was carried out. The photocatalytic activity was evaluated using the photodegradation of methylene blue (MB) dye under UV irradiation. Further, the optical properties of as-prepared ZnO nanoparticles were investigated by UV-vis spectrophotometry. The absence of surfactant led to a simple, cheap and fast method of synthesis of zinc oxide nanoparticles. (C) 2011 Elsevier B.V. All rights reserved.