For last decades scientists have been interested in photocatalysts due to their possibilities to degrade organic compounds into less toxic compounds. In this research ZnO nano and micro particles were obtained in 3 synthesis series. In every series different substance (urea (A series), sodium bicarbonate (B series), sodium carbonate (C series)) was added to zinc nitrate hexahydrate. The temperature was changed in range 100oC to 225oC for each series to determine the best synthesis conditions for microwave - assisted synthesis of ZnO photocatalysts. Microwave-assisted synthesis method was chosen because it is faster than other methods and the process can be made under high pressure conditions, which ensures particles with smaller crystallite sizes and grater specific surface area (BET). After synthesis samples were filtered, grinded and heated at 400oC for 2 hours. Photocatalytic activity of each sample was determined by using spectrophotometric method and degradation of MB (methylene blue) solution for 30min under UV/VIS (300W) irradiation. By evaluating obtained photocatalytic activities results, in Fig.1 is shown every series most active sample. The highest result was shown by A series sample, which was obtained at 125oC, B series sample which was obtained at 200oC and C series sample which was obtained at 225oC. They can degrade 92.2%, 98.0% and 98.8% of MB in 30min, respectively. In conclusion the highest photocatalytic activity is shown by C sample. It is determined that sample A average crystallite size is 46nm, B sample-35nm and C sample-24nm, which allows to conclude that sample with the smallest crystallite size has the highest photocatalytic activity. By evaluating specific surface of the samples, it is concluded that C sample has shown the highest specific surface-38.3m2 /g, while A sample and B sample showed 23.9m2 /g and 34.4m2 /g, respectively. By performing XRD analysis it is concluded that every sample contains only ZnO zincite crystalline phase.