IPP hollow-fiber microporous membranes were prepared via thermally induced phase separation (TIPS) with co-solvent di-n-butyl phthalate (DBP) and dioctyl phthalate (DOP). The DOP mass fraction in co-solvent (alpha) and iPP mass fraction in a casting solution (beta) were worked as the variables in the spinning process. With increasing alpha, the TIPS of the polymer solution changes to only polymer crystallization from liquid-liquid phase separation with subsequent polymer crystallization. Accordingly, the morphology of the resulting membrane changes from a typical cellular structure to a mixed structure, which is basically cellular but with particulate boundaries, and then a typical particulate structure. As a result, permeability of the membrane increases sharply and mechanical properties of the membrane decrease obviously. Therefore, the iPP hollow-fiber microporous membrane with higher permeability and higher mechanical properties must exhibit a mixed membrane morphology in which liquid-liquid phase separation precedes polymer crystallization. However, by varying beta, both the phase separation pattern of the iPP solution and types of the resulting membrane structure cannot be changed. The permeability of resulting membrane decreases with increasing beta, but the mechanical properties increase with increasing beta. It is noted also that those iPP hollow-fiber microporous membranes are apt to possess a narrow pore size distribution. It is indicated that by choosing proper alpha and beta, the membrane morphology can be an open cellular pore structure; moreover, the resulting membrane exhibits both higher permeability and higher mechanical properties. It is suggested that for a crystalline polymer such as iPP, by a proper approach, adjusting the competition between liquid-liquid phase separation and polymer crystallization is the key to creating the membrane with an interconnecting pore structure and good performance.