The gas response of tungsten trioxide (WO 3) based sensors strongly depends on the surface properties. Reconstructed surfaces and oxygen point defects at the surface of the monoclinic WO 3 are studied using a self-consistent scheme based on first-principle. The oxygen vacancy is found to be the predominant defect independently of the oxygen partial pressure. Indeed, under rich oxygen atmosphere the formation enthalpies are found to be 1.45 eV in LDA (1.28 eV in GGA) for the oxygen vacancy instead of 2.70 eV (2.42 eV) for the oxygen adatom. When the oxygen partial pressure is lowered, the oxygen vacancy formation enthalpy decreases and becomes exothermic under very O-poor condition (-1.65 eV in LDA and-1.36 eV in GGA). On the other hand, the formation enthalpy of an oxygen adatom rises. Finally, the oxygen vacancy formation acts as a n–doping by introducing negative charge carriers at the bottom of the conduction band. All these results can be very helpful in order to explain the electrical resistivity measurements.