In this paper, p-type ZnO powder was prepared by decomposition of zinc nitrate at 350 °C for 1 h. p–n junction photocatalyst p-ZnO/TiO 2 was prepared by ball milling of TiO 2 in H 2O solution doped with p-ZnO. The p–n junction photocatalyst p-ZnO/TiO 2 was characterized by UV–vis diffuse reflection spectrum, scanning electron microscopy (SEM), X-ray powder diffraction (XRD) and the fluorescence emission spectra. The photocatalytic activity of the photocatalyst was evaluated by photocatalytic reduction of Cr 2O 7 2− and photocatalytic oxidation of methyl orange (MO). The results showed that the photocatalytic activity of the p–n junction p-ZnO/TiO 2 is much higher than that of TiO 2 on the photocatalytic reduction of Cr 2O 7 2−. However, the photocatalytic activity of the photocatalyst is much lower than that of TiO 2 on the photocatalytic oxidation of methyl orange. Namely, the p–n junction photocatalyst p-ZnO/TiO 2 has higher photocatalytic reduction activity, but lower photocatalytic oxidation activity. When the amounts of doped p-ZnO are 0.0 and 2.0 wt.%, illumination for 20 min, the photoreduction efficiencies are 15.7 and 42.8%, and the photooxidation efficiencies are 68.1 and 26.1%, respectively. Effect of ball milling time on the photocatalytic activity of the photocatalyst was also investigated. The mechanisms of influence on the photocatalytic activity were also discussed by the p–n junction principle.