Abstract: Anatase TiO₂(001) surface arouses lots of research interests since it is believed to be the most reactive surface. However, recent STM measurements showed that except the defect sites, anatase TiO₂(001) (1×4) reconstructed surface is inert to H₂O adsorption. It was indicated that oxidation could be the reason which induces the inert surface reactivity. Therefore, it is strongly motivated to understand the oxidation structures as well as the oxidation process on this surface. In this work, based on first principles calculations, we investigated the oxidized structures and processes of TiO₂ anatase (001) surface with (1×4) reconstruction. We have discovered two kinds of oxidized structures through the molecular adsorption and dissociated adsorption with different oxidation ratio. To understand the oxidation process, we studied the reaction barrier of oxidation process. We conclude the stability of different oxidized structures with different oxidation ratio by comparing the free energy of the system as a function of oxygen chemical potential. Based on that, a first-principles-based phase diagram of the low-energy oxidized surface structures is provided. The effects of the lattice stress are also studied. Results show that the oxidized structure and oxidation ratio strongly depend on the temperature and pressure. The lattice stress also plays an important role.