Five surfaces with low Miller indices, including (001), (011), (111), (01 1) and (11 1), were generated by cleaving rhombohedral Ti2O3, and their surface energies were compared, which showed that (011) is cleaved more easily than others, followed by (001), (111), (01 1) and (11 1), so that we focused on (011), (001) and (111) surfaces. Phase diagram of stoichiometric, oxygen-deficient and nitrogensubstituted Ti2O3 (001) surfaces indicated that oxygen evaporation does not take place on stoichiometric surface, and oxygen-to-nitrogen substitution is not a indirect (two-step) process (oxygen evaporation, followed by nitrogen substitution into oxygen vacancies). It was proved from minimum energy path (MEP) and DFT total energy curves over MEP that the energy barrier of direct oxygento- nitrogen substitution process is lower than indirect process, and therefore, direct process is easier to take place than indirect process.