The effects of nucleating agents and extractants on the microstructure of polypropylene (PP) membranes via thermally induced phase separation were investigated. The nucleating agents were dibenzyl sorbitol, adipic acid and benzoic acid. The extractants were methanol, ethanol, formic acid, acetone, butane, n-hexane, dichloroethane and xylene. The spherulitic size of the membrane greatly decreased and the degree of crystallization and enthalpy of crystallization remarkably increased when adding dibenzyl sorbitol; therefore, dibenzyl sorbitol was a perfect nucleating agent for a PP/soybean oil/dibutyl phthalate (DBP) system. Moreover, with the addition of 5 wt% of dibenzyl sorbitol, the membrane had the smallest spherulitic size. It was also proved that ethanol, acetone, butane, n-hexane and dichloroethane are proper extractants for a system of PP/DBP/soybean oil.