Abstract: The specific process of membrane formation via water-vapor induced phase separation method was investigated through observing the gelation process of polyvinylidene fluoride(PVDF)casting-solution in vapor under optical microscope,combining mass variation of the membrane solution under different humidity and infrared spectra of the liquid drop layer on the film surface.The results showed that there existed transfer between solvent and non-solvent during the membrane formation.The film quality increased in the beginning due to the absorption of non-solvent water as dominant.Afterwards,film quality reached a maximum value,then decreased gradually to be steady owing to the volatilization of the solvent and non-solvent as dominant.The gelation rate in the high humidity was faster than that in the low humidity because of the faster exchange between the solvent and non-solvent.According to the SEM images of the PVDF membrane surface and cross-section,the final membrane structure for rough and porous surface was formed with the longer exposure time in vapor while the symmetrical cross-section formed.