Numerical simulations of phase separation in Fe-Cr-Mo?Ni or Fe-Cr-Mo-Ti quaternary alloys similar to ferrite phases in duplex stainless steles were performed by the Cahn-Hilliard equation. We obtained that the asymptotic behavior of minor element Ni, Mo, Ti in an Fe-Cr-X quaternary alloy along a trajectory of a peak top of the major element Cr is 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(X=Mo, Ni or Ti]. It is also predicted that that small addition of Mo up to 10% accelerates phase separation of Cr. It seems that the optimum value of Mo exists to enhance the phase separation of Cr in Fe-Cr-Mo ternary alloys. Simulation result indicates that the phase separation of Cr is most enhanced with addition of 1% Mo. The above mentioned simulation results given by the numerical simulation by the Cahn-Hilliard equation were in good agreement with those obtained by the Monte Carlo simulation Theoretical analyses were performed in order to discuss the simulation results. On the basis of theb simulation results Optimum materials design of the duplex stainless steel has been established.