Phase equilibria among the bcc Fe(α), fcc Fe(γ) and Fe2Mo(λ)_phases in Fe-Mo-Ni ternary system, particularly paying attention to the existence of the γ+λ two-phase region, have been examined at elevated temperatures below Tc (1200 K), the peritectoid reaction temperature in Fe-Mo binary system: λ?α+Fe7Mo6 (μ). At 1173 K the α+γ+μ three-phase coexisting region exists near the Fe-Mo binary edge and no λ phase region was identified. At 1073 K the λ phase in equilibrium with α and γ phases exists, although the composition homogeneity region of the ternary λ phase was limited to its binary edge toward the equi-nickel concentration direction up to about 3at % Ni. Instead, large two-phase region of γ+μ was extended along the same direction up to 20 at% Ni. The γ+λ two-phase region appears below Tc through a transition peritectoid reaction: α+μ¨γ+λ. The γ phase in equilibrium with λ phase is stable only at elevated temperatures, and it transforms martensitically to α phase during cooling. The addition of Ni stabilizes γ and μ phases against α and λ phases, thereby decreasing the relative stability of the λ phase.