In this paper, efficacy of p-n junction p-CuO/n-ZnO composite is assessed as a potential photocatalyst by monitoring degradation of methylene blue (MB) in the presence of UV light. The p-n junction photocatalyst, p-CuO/n-ZnO, was prepared by ball milling of ZnO and CuO in water. The structural properties of p-CuO/n-ZnO composite were characterized by x-ray diffractometer and surface charge properties via zeta potential measurement. The degradation of MB in the presence of composite powder was monitored via UV-vis spectrometer. Various studies affecting the degradation rate of MB were conducted as a function of weight fraction of CuO in the composite and ball milling time. The highest degradation rate of MB was achieved in CuO (10 Wt.%)/ZnO for which high negative zeta potential was recorded. The MB degradation efficiency was found to decrease with the samples ball milled for time longer than 12 hours due to increased agglomeration of particles. The mechanisms that influence the photocatalytic activity of p-CuO/n-ZnO are discussed based on the p-n junction principle.