We report the synthesis of Zn1−xCaxO nanoparticles prepared by a sol–gel processing technique for x ranging from 0 to 0.05. The structural, morphological and optical properties of the nanoparticles were investigated by X-ray diffraction, transition electron microscopy, UV measurements, and photoluminescence spectroscopy. The photoactivity of the Ca-doping ZnO photocatalysts was evaluated by the photocatalytic degradation of methylene blue dye. The structural properties showed that the obtained nanoparticles are in wurtzite single crystalline phase and no secondary phases were detected which indicated that Ca substituted Zn ions. The absorption edge of the prepared photocatalysts is clearly shifted to the visible region as compared to undoped ZnO. The study of the effect of Ca loading on the photocatalytic activity shows that the Zn0.96Ca0.04O is the most photoactive. The effect of some parameters such as catalyst loading, initial dye concentration and light intensity were investigated.