The existing forms of the impurity Nb5+ in the Nb5+:PbWO4 (PWO) crystals are simulated by computer technology. The possible various kinds of defects in the Nb5+:PWO crystals are also simulated. By analyzing the calculation results of defect formation energies and binding energies, the optimal substitution positions of the Nb5+ ions and the charge compensating mechanism [Nb-w(-)+V-0(2+)+Nb-w(-)] in the Nb5+:PWO crystals were obtained. The electronic structures for Nb5+:PWO were calculated with density functional theory code CASTEP. It shows that the doping of Nb5+ ions could suppress the 350 nm absorption band. (C) 2009 Elsevier B.V. All rights reserved.