Numerical simulations of phase separation in Fe-Cr-Mo and Fe-Cr-Ti ternary alloys and Fe-Cr-Mo-Ti quaternary alloys were performed with use of the Cahn-Hilliard equation for ternary alloys and quaternary alloys. We obtained that the asymptotic behaviours of minor element X(Mo, Ti) in Fe-Cr-X ternary alloys and Fe-Cr-Mo-Ti quaternary alloys along a trajectory of a peak top of the major element Cr was classified into three groups according to the sign of the second derivative of the chemical free energy with respect to the compositions of Cr and X(Mo, Ti). Theoretical analysis for the symptotic behavior Mo in Fe-Cr-Mo ternary has been formed in order to discuss the simulation results. Effect of the other elements, such as Ti on separation behaviours of Cr in Fe-Cr-Ti ternary alloys were also investigated. A simple theory for describing the effect of subsutitutonal element in Fe-Cr-X ternary alloys on the basis of simulations and theoretical analyses was proposed.