Crystalline ZnO photocatalytic nanoparticles were prepared by calcining the polyol-derived solid precursor in air at T400 degrees C for 3h. The polyol-derived solid precursor was prepared by isothermally treating the ethylene glycol solution of zinc nitrate at 160 degrees C for 8h. The resultant suspension was filtered, washed, dried and calcined to form the required oxides. The photocatalytic ability of the ZnO particles was investigated by measuring the ability of the calcined ZnO particles to photocatalytically decompose the methylene blue (MB) in water, under 365nm UV light irradiation. The ZnO particles calcined at 400 degrees C T700 degrees C exhibited better photocatalytic abilities than the P25. The photocatalytic kinetics for the MB decomposition reaction were investigated by estimating the corresponding specific reaction rates, such as (based on mass of the catalyst used) and (based on surface area of the catalyst used). It was found that the specific surface area and the crystallinity of ZnO particles strongly affected the performance of ZnO photocatalysts. The ZnO particles calcined at 500 degrees C had an optimized combination of the crystallinity and the specific surface area, and gave the best photocatalytic ability per unit particle mass.