Abstract: The liquid-liquid phase inversion method is used for preparation of polyurethane(PU) microporous membranes and the mechanism for the formation of micropores is investigated. Influence of nonsolvents on the fabrication of microporous thermoplastic PU membranes is studied by experimental research and theoretical analysis. Surface and cross-section morphologies of PU membranes are studied by scanning electron microscopy (SEM). SEM photomicrographs show that PU membranes prepared in alcoholic coa-gulant have uniform porous structure compared to those prepared in water coagulant. A uniform inter-connective porous structure is successfully prepared using isopropanol as the nonsolvent. The Flory-Huggins interaction parameter and diffusion coefficient of the four kinds of nonsolventsincluding water, methanol, ethanol and isopropanol are calculated and discussed in combination with the membrane formation mechanism.