The metallic nickel (Ni) deposited on an n-Si substrate with resistivity of 4 – 6 Ω∙cm was oxidized by the ultra-violet (UV) oxidation technique to form a p-NiO/n-Si heterojunction diode. The rectifying current-voltage (I-V) characteristic confirmed formation of a pn junction. The capacitance-voltage (C-V) characteristic further identified an abrupt p+n junction between NiO and n-Si. The photocurrent increased with the increased wavelength of laser under illumination of the diode. The voltage-dependent photocurrent suggests that the carriers generated in the depletion region of Si was effectively collected but not outside the depletion region. A low diffusion length of holes was attributed to Ni diffusion in Si caused by the substrate heating during the UV oxidation.