Ionic liquid was used as the pore-forming additive to design the structure and properties of PVDF membranes. The crystal structure, morphology and the wettability of the membranes was investigated by Fourier transform infrared spectrometer (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM) and contact angle measurement. The results of FTIR and XRD suggested that ionic liquid could make α crystal phase in pure PVDF membrane totally transformed into β crystal phase via immersion precipitation process. SEM results also indicated that ionic liquid was a good pore-forming additive, which was in favor of the formation of the pore structure in the membranes. Contact angle results showed that the surface hydrophobic of PVDF membrane increased with the addition of ionic liquid in the casting solution. These results suggested that ionic liquid may be found its further application in the preparation of piezo-electric PVDF films with β crystal phase, and oil/water separation membranes.