Numerical simulations of phase separation in Fe–Cr–Mo ternary alloys were performed with use of a model based on the Monte Carlo simulation, in order to investigate mechanisms of phase separation in Fe-based ternary alloys. Cr-rich regions were formed in an Fe–40at.%Cr–5at%Mo alloy. Mo atoms enrich into the Cr-rich region and/or boundaries of Cr/Fe rich regions. Behaviors of Mo and Cr in an Fe 40at.%Mo–5at.%Cr alloy were similar to those in the Fe–40at.%Cr–5at.%Mo alloy. The first peak position of the structure factor moves on to the shorter side of the wave number with the increase of temperature. Analysis of the static structure factor of a minor element indicates that the bifurcation formation of concentration profile of the minor element occurs at peak positions of the major element which is predicted by a theory based on the Cahn-Hilliard equation.