The structure, porosity and crystallization behavior of poly (L-lactic acid) and poly (L-lactic acid)/polyurethane porous membranes, prepared from ethanol/dioxane and ethanol/water coagulation baths through immersion precipitation, have been systematically investigated. The diffusion rate between solvent and nonsolvent as well as the equilibrium phase diagram of PLLA/solvent/nonsolvent system were also well studied. It has been proved that the ultimate structure and performance of the membranes could be mediated under control by suitable adjustment on phase separation behavior of the ternary system through varying coagulation bath compositions. The results show that the presence of lower ratio of dioxane in ethanol baths endows the resulting membranes with uniform sponge-like structure, higher porosity and crystallinity due to the moderate solidification and crystallization of PLLA, while increasing the water concentration tends to have a modestly opposite effect and obtains membranes with irregular finger-like structure, lower porosity and crystallinity. Under the same coagulation baths, PLLA/PU membranes possess slightly larger pores size and porosity than pure PLLA membranes, but the presence of PU appears to have no effect on the crystallinity of PLLA. (C) 2013 Elsevier Ltd. All rights reserved.