A new technique has been established to fabricate thin him composite membranes, by which a hydrophilic polymer could be coated in thin film on a hydrophobic support membrane. The new technique was composed of two steps: dispersion of a reactant to the hydrophilic polymer in the hydrophobic support membrane and interfacial reaction between the reactant and the hydrophilic polymer to produce thin film of the hydrophilic polymer on the support membrane. Composite membranes in which a thin film of sodium alginate is coated on a polysulfone support membrane were prepared by the new technique for the reverse osmosis separation of anionic surfactant-water mixture. Two methods were employed to fabricate a thin film of sodium alginate on the support membrane: (1) dispersion of the crosslinking agent, CaCl2 alone in the support membrane and (2) dispersion of CaCl2 in the support membrane with help of PVA which adheres fast to the support membrane. The formation mechanism of the thin layer was suggested schematically on each method. Both the methods could produce successively a thin layer of SA on the support membrane. Especially, method (2) gave a strong bonding of the thin layer on the support because of the large contact area with the support through the PVA layer which sticks fast to the SA layer. From the SEM pictures and permeation experiments, the method (2) was confirmed to be better to produce a defect-free thin film of SA on the support membrane.