In this study, nonsolvent was added in casting solution to elevate the porosity of asymmetric TPX (poly(4-methyl-1-pentene)) membranes, prepared by the wet phase inversion method. In addition, to prevent the shrinkage of membranes during the formation process, the dope (TPX/cyclohexane/nonsolvent) was evaporated for 30 s before immersing in the coagulation bath (ethanol). Both coagulation value and solubility-parameter difference were proved to be good criteria for choosing suitable nonsolvent additives. Further, it was found that the addition of nonsolvent can have drastic effect on membrane morphology. Three kinds of membrane structure were observed : sponge, finger (macrovoid) and "cellular surface". Our experimental results indicate that macrovoids are easy to grow in case of delayed demixing and that the formation of the "cellular surface" structure is strongly related to the interfacial instability induced by evaporation.