Three dimensional microstructures resulting from phase separation in thermally aged Fe-Cr-Mo ternary alloys were investigated by using Monte Carlo simulation and numerical solution of the Cahn-Hilliard equation. The Monte Carlo simulation result indicates that Cr rich regions were formed in an Fe-40at.%Cr-5at.%Mo alloy and that Mo atoms enriched into the Cr rich region and/or boundaries of Cr/Fe rich regions. Behaviors of the major and minor elements, Mo and Cr, in an Fe-40at.%MO-5at.%Cr alloy were similar to those of Cr and Mo in the Fe-40at.%Cr-5at.%Mo alloy. The first peak position of the structure factor of a major element moved to the shorter side of the wave number with the increase of aging 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.