The membrane formation mechanism of PVDF／DMAc／H2O／Additive svstem was studied．The additive was PVP

PEG or LiCl respectively. The phase diagram for polymer solution of PVDF／DMAc／Water/Additive system was measured using cloud-point method. The phase separation phenomena during membrane formation process were recorded using light transmission experiment．The crystallinity of membrane was detected by DSC．The result shown both gelation and liquid-liquid demixing existed during phase-inversion process of casting solution with different additive．Gelation appeared early at lower nonsolvent concentration than liquid-liquid demixing in the range of 30～60℃. LiCl as additive lead to a strong action for PVDF solution to form gel than PEG or PVP．This incteased the mass transfer resistance in nascent membrane and the liquid-liquid demixing delayed time prolonged along with the sequence of PVP

PEG or LiCl as additive during membrane formation process．The polymer concentration of nascent membrane at the onset of liquid-liquid demixing increased and restricted the development of macrovoids．The strong gelation tendency of polymer solution intensified the solidification of nascent membrane and restricted the macrovoids development further．Thus the macrovoids structure extended to the bottom and stopped with the sponge-Iike structure when PVP was used as additive．When the additive was PEG or LiCl．the macrovoids developed insufficiently and terminated in the middle of membrane with the sponge-like structure and then passed to nodular structure in bottom．The effective porosity and flux decreased and crystallinity increased along with the sequence of PVP

PEG or LiCl as additive．The hydrophobicity of membrane with LiCl as additive was barely changed

but the water-resisting pressure of membrane with PVP or PEG as additive decreased about 20 kPa．