NiO is a transparent semiconductor oxi de with p-type conductivity,having a direct energy gap of 3.7eV,with weak abso rption bands due to d-d transitions of 3d electron configuration in the visible region.Here,NiO films were prepared on n type Si substrates by magnetron sputte ring to form Si/NiO pn heterojunctions.The XRD results show that only one diffra ction peak appear for NiO films on Si substrates,which is corresponding to (111) NiO preferred orientation and indicates that NiO belongs to cubic structure.The right shift of (111) NiO peaks indicates the depression of c axis distances,whi ch may result from the relaxation of strain between NiO and Si substrate and the decrease of defects.The transmittances of NiO films have been obviously improve d with annealing temperature,which means annealing temperature is benefit to the better crystallization and the decrease of crystal defects,which has been evide nced by AFM pictures.From I-V curves,the electrical properties have changed bet ter when the annealing temperatures increase and the best rectifying property sh ows in the Si/NiO pn junction annealed at 600℃,where the rectification ratio ( I(1+7V)/I(－7V)) is 12and its Voc arrives t o 3.5V and the leakage current can be observed until the negative voltage reach es －7V.Considering the results of XRD,AFM,UV and I-V,it can be concluded tha t the increase of annealing temperature is beneficial to the crystal structure a nd reduces the crystal defect,the increase of transmittance in NiO films and the improvement of the electircal peroperties of Si/NiO pn junctions.