Nanostructures of Zn1-xMgxO (0 <= x <= 0.2) were prepared in water by one-pot method under microwave irradiation for 5 min. In this method, zinc acetate, magnesium nitrate and sodium hydroxide were used as starting materials without using any additive and post preparation treatment. The nanostructures were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray (EDX), X-ray photoelectron spectroscopy (XPS), diffuse reflectance spectroscopy (DRS), electrochemical impedance spectroscopy (EIS), Fourier transform-infrared (FTIR), and the Brunauer-Emmett-Teller (BET) techniques. The nanostructures have wurtzite hexagonal crystalline phase and doping of Mg2+ ions does not change the phase of ZnO. The SEM and TEM images show that morphology of the samples is changing by doping of Mg2+ ions. The EIS data show that by doping the ion, interfacial charge transfer resistance of the nanostructures decreases. Photocatalytic activity of the nanostructures was evaluated by degradation of methylene blue (MB) under UV irradiation. The degradation rate constant on the nanostructures with 0.15 mol fraction of Mg2+ ions is about 2-fold greater than for ZnO. Moreover, influence of various operational parameters such as microwave irradiation time, calcination temperature, weight of catalyst, concentration of MB, pH of solution and scavengers of reactive species on the degradation rate constant was investigated and the results were discussed. (C) 2014 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.