The influence of niobium (Nb) doping on the properties of lead tungstate crystals (PWO) has been investigated. Transmission, emission, positron annihilation lifetime spectrum and X-ray photoelectron spectra(XPS)of PWO∶Nb and undoped PWO were reported. The luminescence characteristics of the Nb-doped PWO crystal was discussed by comparing the results with that of pure PWO crystal. Nb doping improved the luminescence property of PWO and enhanced the percentage of its quick component. The defects in PWO crystal caused by Nb dopant has been studied by means of positron annihilation lifetime spectrum and X-ray photoelectron spectra. The results show that Nb dopant increases the concentration of lead vacancy (V Pb) as well as the concentration of low-valent oxygen. The Nb 5+ will occupy the lattice of W 5+ and make some (WO 4) 2- change to (NbO 3+V O)-, which will result in the improvement of luminescence performance of PWO crystal.