Nano Co3O4 - CuO - ZrO2 mixed oxides were synthesized by wet chemical method by mixing of equimolar solutions of cobalt chloride, copper sulphate and zirconium oxychloride in aqueous sodium hydroxide and refluxed at elevated temperature. The photocatalytic activity of Co3O4 - CuO - ZrO2 NPs was evaluated for degradation of methylene blue (MB) under sunlight. The photocatalytic activity is affected by pH of dye solution, photocatalyst particle size, photocatalyst dosage and dye concentration. The rate of MB degradation is almost 2.49 fold higher in the presence of sunlight than in the absence of sunlight. The degradation efficiency of MB is considerably increased from 78.38% to 88.18% with increasing pH from 4 to 9. 0.1M Co3O4 - CuO - ZrO2 NPs with smaller particle size (12.93nm) exhibits stronger photocatalytic activity as compared to other NPs (0.2M – 0.5M) with larger particle size (13.29 – 23.83nmsize). The dye degradation increases with increasing catalyst dosage, which is characteristic of heterogeneous photocatalysis. While increasing the dye concentration from 1.0 to 2.5 x 10-5 M the rate of degradation decreases.