The relationships between the microstructural and physical properties of the whey protein isolate and gelatin (WPI/gelatin) composite films were investigated in the present work. Through the electrostatic effects at pH 8, WPI and gelatin molecules could form compact aggregates in solution, where a remarkable shrinkage of the gelatin molecules was observed, when the WPI/gelatin mass ratio was close to 50W:50G. FT-IR analysis indicated that hydrogen bonding also involved the aggregation and film-forming process. The melting temperature of the 50W:50G composite film increased by 9 degrees C compared with the single component films. However, this aggregation process also made the film network microstructure discontinuous, and led to a decline of the puncture strength of the film near 50W:50G; in contrast, the deformation and water vapor permeability of the composite films increased with the gelatin content, while the moisture content and solubility did not show significant variations.