Abstract: The "voids" in the CA-CTA blend membranes prepared ttnder different casting conditions have been experimentally observed with a general optical microscope,magnifying 80x.At the same time,the properties for reverse osmosis of the prepared blend membranes have also been studied.It was shown from the experimental results that:(1)when we slowed down the spreading rate of solution in casting,or decreased the casting temperature and increased the time of solvent evaporation,the elimination of voids was favored;(2)the composition of casting solution showed a significant effect on the number of voids:when we decreased the relatve amount of dioxane,added in acetic acid instead of malefic acid,or increased the amount of methanol and acetic acid in the casting solution,the number of voids was decreased;(3)when the temperature of the gelling bath was decreased or acetic acid was added in the gelling bath,the number of voids was decreased;(4)after the membranes have been subjected to heat treatment,no apparent change in the number of voids was observed;(5)When we selected suitable additives in the casting solution and adequate gelling conditions to eliminate the voids almost completely,the desalination properties of the membrane could be improved.