P-N junction photocatalyst NiO/TiO2 photoexcited by visible light was prepared by sol-gel method using Ni(NO3)2•6H2O and tetrabutyl titanate [Ti(OC4H9)4] as the raw materials. The p-n junction photocatalyst NiO/TiO2 was characterized by UV-Vis diffuse reflection spectrum, fluorescence spectra (FL), X-ray powder diffraction (XRD) and scanning electron microscopy (SEM). UV-Vis spectrum showed that the absorption wavelength range of NiO/TiO2 is extended to the visible region, making it an effective photocatalyst photoexcited by visible light. This was demonstrated by photocatalytic reduction of Cr2O72- and photocatalytic oxidation of methyl orange (MO) under visible-light (λ>400 nm) irradiation. Doped-NiO effectively suppressed the phase transformation of anatase to rutile and the growth of titania crystallites. The optimum percentage of doped NiO is 0.5% (mole ratio of Ni/Ti). Effects of heat treatment on the photocatalytic activity of p-n junction photocatalyst NiO/TiO2 were investigated. The mechanisms of influence on the photocatalytic activity were also discussed by the p-n junction principle.