Abstract: The UF and RO membranes performances of cyanoethyl cellulose acetate (their viscoses are 340 and 70 cp respectively) were studied and compared with those of cellulose acetate membrane.The results show that the membrane performance of cyanothyl cellulose acetate is least affected even if its viscose is changed about 5-fold, but greatly affected by the purity and molecular weight. The flux of cyanoethyl cellulose membrane is higher than that of the CA, for the cyanoethyl is a hydrophilic radical. The casting constituents and technologic conditions were also studied. If some alcohol or dioxane is used as secondary additive and 2% washing detergent is used as dipping agent, the change of membrane rejection is least and the flux is raised.