A simple “green” method was applied for preparation of Zn 1 – x Mn x O nanoparticles ( x = 0, 0.025, 0.05, 0.075, 0.15 and 0.20) in water by refluxing for 2 h at about 90 °C. This method is mild and highly efficient involving no templates, surfactants or additives; therefore, it is very promising for low-cost and large-scale production. Powder X-ray diffraction patterns demonstrate that the nanoparticles have the same crystal structure, and loading of Mn 2+ ions does not change the structure of ZnO. Scanning electron microscopy images show that the nanoparticles are mainly spherical. UV-Vis diffuse reflectance spectra of the samples demonstrate increasing visible light absorption with mole fraction of Mn 2+ ions. Photocatalytic degradation of methylene blue on the nanoparticles was evaluated under visible light irradiation. Among the prepared nanoparticles, Zn 0.95 Mn 0.05 O has the highest photocatalytic activity. Moreover, effects of refluxing time and calcination temperature on photocatalytic activity were investigated.