A conception of critical solvent concentration (CSC) at the onset of phase separation was proposed. The macrovoid-formation mechanism came up with this conception．It was considered that a higher solvent concentration at lhe onset of phase separation than that critical value is required for the development of macrovoids in phase-inversion membranes. When the solvent concentration at the onset of phase separation is smaller than the critcal value

the macrovoids development stops and a sponge-like structure is fonned．The polyvinylidene difluoride(PVDF) hollow fiber membranes by using PVDF(18％)-PEGl000(5％)-DMAc(77％)casting solutions and different inner coagulation baths．A mass transfer model was established to simulate the composition variation of nascent hollow fiber during the PVDF hollow fiber formation process under the corresponding technical condition. The solvent concentration at the onset of phase separation was calculated．The membrane morphological structures were predicted according to the conception of CSC at the onset of phase separation

the macrovoids formation mechanism and the simulation resuIts. SEM observations of the membranes prepared in various inner coagulation baths show that the predicted results coincide well with the membrane morphological struetre and verify the macrovid-formation mechanism．