The ultrathin membranes of cellulose acetate and polyvinyl chloride were prepared by the method of watercasting. The spreading ability of polymer solutions on water doereased with increasing the surface tention and the viscosity of polymer solutions, and the temperature of water surface, respectively. The aggregation states of water-casting membranes were dense and dense with increasing the concentration of polymer solutions and the temperature of water surface. The surface structure of the air sides showed more dense than that of the water sides. In the case of the 3 wt% cellulose acetate/acetone system, the thickness of the membrane was about $0.1{mu}.$