Permeation of water and various water soluble solute through copoly(γ-benzyl-L-glutamate-L-leucine) membranes was investigated. L-Leucine NCA and γ-BLG NCA synthesized by phosgenation of L-Leucine and BLG were polymerized homogeneously at 30℃ in dioxane using triethylamine as an initiator. Copolypeptide membranes were casted from dioxane or benzene solution on a glass plate. It was found that prepared membranes gave α-helical structure. The measurement of water permeability and solute separation of membranes were carried out by using a batch-type permeation apparatus. Water flux through copolypeptides was smaller than through homopolypeptides, but rejection by copolypeptides was greater than by homopolypetides''. The contact angle and clotting time of membranes were investigated in order to examine the thromboresistant property of copolypeptide membranes. Three components of the surface tension were estimated from the measurement of contact angle. The segment of L-leucine residue was found to be better blood compatible than the segment of γ-BLG residue in copolypeptides, and the clotting times were affected by surface tension and hydrogen bond in the copolypeptide units.