A series of segmented polyurethanes (SPUs) using poly(tetramethylene oxide) (PTMO), 4,40-methylenebis (phenyl isocyanate) (MDI), and ethylene glycol (EG) with different molar ratios of MDI and PTMO were prepared to study the effects of the hard segment content in SPU on liquid-liquid phase separation in the SPU/dimethylformamide(DMF)/water system. The cloud points were obtained by a titration method and the phase diagrams were calculated based on the Flory-Huggins thermodynamics. The DMF±SPU interaction parameter, χ23 was determined by the intrinsic viscosity measurement. The water-SPU interaction parameter, χ13 was measured by an equilibrium swelling experiment. As the hard segment fraction in SPU increased, the values of x 23 increased slightly and those of x 13 decreased significantly. The amount of water to achieve liquid demixing increased systematically with the hard segment content. The x 13 values in the practical concentration range were estimated by comparison of the calculated phase diagrams with the experimental cloud points. Liquid-liquid phase separation was coupled with liquid-solid phase separation (crystallization) when the SPU had high concentration of the hard segment. These coupled phenomena in combination with the phase segregation in SPU arising from the incompatibility between the hard and soft segments may be utilized to produce a variety of the morphologies of polyurethane membranes prepared by immersion precipitation.