For extracting the salinity gradient energy, membrane is a key material with a high power density. For this purpose, thin film composite (TFC) membranes were synthesized by interfacial polymerization using various acyl chloride and amine monomers. TFC membranes were characterized by FT-IR, SEM, XPS, and AFM. Among the range of TFC, the best candidate was prepared by combining 3,5-diaminebenzonic acid (0.5 wt% with distilled water) and 1,3,5-benzenetricarbonyl trichloride (0.2 wt% with n-hexane). The TFC membrane (thickness = 0.02 mm) generated a power density of 7.2 W/m2 at salinity gradient between 0.005 and 0.5 M KCl solutions.