Based on density functional theory and thermodynamic model, we compile a phase diagram for the polar NiO(111) surface as a function of temperature and oxygen pressure. The electronic correlation between Ni-3d electrons has also been included in the form of GGA+U method. Consistent with recent experiments, present GGA+U calculation indicates that over a broad range of oxygen partial pressure, the most stable phases are the oxygen and Ni terminated octopolar structures, which are almost degenerate in energy. We also show that the stabilization of the NiO(111) surface goes together with remarkable changes in the geometrical and electronic structure.