Asymmetric membranes having smaller pores at one side and larger pores at the other side were prepared by thermally induced phase separation of polypropylene-diphenyl ether solution with polymer concentration gradient. The diluent was evaporated from one side of the polymer solution, thereby creating a concentration gradient prior to the phase separation. The thickness of the skin layer formed near the upper side of the membrane was influenced by the evaporation time. Analysis of the evaporation process indicated that clear polymer concentration gradient formed by the evaporation of diluent. The simulation of phase separation process by the spinodal decomposition was carried out based on Cahn-Hilliard's model. The simulation results of the pores size and the thickness of skin layer were approximately in agreement with the experimental results.