Porous membranes were prepared from a polymer blend system by the thermally induced phase separation (TIPS) process. The polymer blend system was isotactic polypropylene (iPP)/polybutene (PB) and the diluent was diphenyl ether (DPE). Two types of porous membranes were prepared by the extractions of DPE alone and both DPE and PB after the phase separation. The effect of the addition of PB to the iPP solution on the phase diagram was investigated and the phase separation kinetics was measured by the light scattering method. The addition of PB resulted in the higher solute rejection property and lower water permeance. By the further extraction of PB from the porous iPP/PB membrane prepared by the extraction of DPE, the water permeance was approximately doubled, maintaining almost the same rejection property.