Abstract: Theoretical and expermental researches have been made on the hydrodynamic in-stabililty of casting film solution and the formation of an ultrathin membrane. The results show that the hydrodynamic instability of the solution leads to forming holes in the skin layer of the membrane. The solution thickness on the solid support during the ultrathin film casting operation is significantly responsible for hydrodynamic instability. With an optimum thickness of ultrathin membrane, the instability can be suppressed and the holes in the skin layer can be reduced or eliminated, so that the ultrathin composite membrane will have high rejection as well as high water flux.