In this work, the oxygen vacancy in WO3 has been studied by an ab initio pseudopotential method within the local density approximation (LDA). It is shown that with the charge state change of the vacancy, a strong lattice relaxation, swing from one to the other side of the un-relaxed position, is found for the nearest W ions, accompanied by large changes in the electronic structure of the vacancy. It is found that an oxygen vacancy in WO3 gives rise to three types of defect states: a donor-like state near the fundamental band gap, derived from the top valence bands, a hyper-deep resonant state in the valence band and a high-lying resonant state in the conduction band, derived from the oxygen 2s bonding and anti-bonding band, respectively. The existence of the donor-like defect state offers a possible explanation for the dependence of the conductivity and the mid-gap absorption on the O deficiency. (C) 2003 Elsevier B.V. All rights reserved.