The relationship between preciption kinetic effect and morphologies of 6FDA-4,4'ODA polyimide membrane was investigated. The effect of the exchange rate of diffusion of DMAc-additive into the casting polymer solution is explored. It is found that if a nonsolvent contained a lager amount of DMAc is used, polymer solution system have a low nonsovent diffusion coefficient and the less porous structure with a defect free skin. It can be predicted what membrane structure will be formed by observing the precipitation kinetics.