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It is commonly accepted that the drop in mechanical properties of INCONE1718(IN718) after long time exposure at high temperature is a result of coalescence ofγ", γ and the transformation ofγ′ to δ-Ni3Nb phase. Besides the γ′, γ and δ,the precipitation of B.C.C. α-Cr is another factor that affects the mechanical properties of IN718. In this paper, the Thermal-Calc software is used to reveal the thermodynamic condition of α-Cr formation. The calculated sections of the pseudo tary diagrams of Cr-Nb-IN718 and Cr-Fe-IN718 explain why α-Cr particles do not precipitate dispersedly in γmatrix but in the vicinity of δ phase. The selected area diffraction (SAD) of TEM confirms the crystallography relationship of α-Cr with δ is (010) δ// (-110) α-Cr, [100] δ//[111] α-Cr° In combination of TTT diagram with the Thermal-Calc and TEM results, the mechanism of α-Cr formation in IN718 is suggested.