The flat sheet nitro cellulose (CN)-cellulose acetate (CA) blended membranes were prepared via phase-inversion method from the casting solutions composed of CN-CA blended polymers and acetone-formic acid mixed solvents. Several membrane preparation parameters were introduced on the membrane fabrication process, and their effects on the morphology and performance of resulting membrane were investigated. The membranes obtained had asymmetric structure composed of an extremely thin dense surface layer (skin layer) on the top of a relatively porous substructure with sponge or finger-type. Finger-type structure was obtained in a membrane prepared with only CN. Sponge-type structure appeared in a membrane prepared with CN and CA. With the increase of the solvent evaporation period for the cast film up to about 2 minutes, water permeability (water flux) was decreased. and then increased with the evaporation period afterwards. On the other hand, solute retention showed opposite result which was an initial increase followed by a decrease with the increasing solvent evaporation periods. NA(5,5) -0.5 membrane had the molecular weight cut-offs more than 10,000. From the result of NA(6,6) membranes, it was suggested that it would be possible to fabricate a membrane capable of effectively repenting a sort of materials with the molecular weight of thousand unit.