The　rule　of　oxygen　potential　increasing　with　oxygen　mole　fraction　in　oxides　and　calculation　with　computer　are　applied　to　assess　the　data　of　standard　Gibbs　free　energy　of　formation　ΔfG0　for　24　types　of　Ti-O　compounds　obtained　from　literatures.　The　results　show　that　the　relationship　of　oxygen　potential　with　the　equilibrium　reactions　of　Ti26O51　with　Ti28O55　and　Ti24O47　with　Ti26O51　do　not　obeythe　rule　of　oxygen　potential　increasing　at　1304　K.　It　is　suspected　that　one　of　ΔfG0　data　of　Ti24O47,　Ti26O51,　and　Ti28O55　is　error.　The　methods　of　evaluating　ΔfG0　by　both　the　oxygen　potential　increasing　and　quasi-parabola　rules　are　explained　in　detail.　Then　ΔfG0　data　of　Ti24O47,　Ti26O51,　and　Ti28O55　are　separately　evaluated.　The　results　obtained　by　the　two　different　methods　are　the　same.　At　1304　K　they　are　ΔfGTi24O470((16785.366(0.011　kJ/mol,　ΔfGTi26O510=-18202.261(0.021　kJ/mol,　ΔfGTi28O550=-19619.080(0.008　kJ/mol.　These　results　are　analyzed,　which　shows　that　the　original　value　in　ΔfG0　of　Ti26O51　is　probably　larger　error.　Based　on　the　assessed　data,　Ti4O7　material　is　prepared　from　TiO2　by　carbothermal　and　hydrogen　reduction　methods.