Mechanical, swelling, and optic properties of composite films prepared from soy protein isolate (SPI) and gelatin were investigated. With increasing gelatin ratio in composite films, tensile strength (TS), elongation to break (EB), elastic modulus (EM) and swelling property of the SPI/gelatin composite films increased. In addition, the films became more transparent, and easier to handle. When the ratio of SPI:gelatin was 4:6–2:8, the TS, EB, and other properties of composite film approached those of gelatin film and were better than those of SPI film. Particularly, the composite film was more economic than gelatin film, so it could be used as edible film instead of gelatin film for package. When the ratio of SPI:gelatin was 4:6, the influences of concentration of glycerin, pH value of SPI film-forming solution, thermal-treatment temperature of SPI film-forming solution and NaCl on mechanical and optic properties, and water content of composite films were also studied.