Two kinds of aqueous dispersions were prepared from waterborne polyurethane (WPU) and its prepolymer by mixing (M) and grafting (G) with 10 wt % casein, respectively. The resulting dispersions were characterized by using a rotary viscometer and a particle size distribution meter, indicating that the viscosity, Z average size, and polydispersity of the dispersions increased with an increase of the casein content, and the average particle size of the M dispersions (ca. 30-50 nm) was much lower than that of the G dispersions (ca. 50-150 nm). Polyurethane/casein composite sheets were prepared from the dispersions M and G, coded as sheets M and G, respectively. The structure and properties of the sheets were characterized by using infrared spectroscopy, scanning electron microscopy, ultraviolet spectrophotometry, dynamic mechanical analysis, and tensile testing. The results revealed that the sheets M and G all exhibited a certain degree of miscibility, and their tensile strength was much higher than that of the sheet of pure WPU. When the casein content was 6 wt %, the tensile strength (sigma(b)) and elongation at break (epsilon(b)) of the G sheet in a dry state achieved 30.1 MPa and 775%, respectively. When the casein content was 4 wt %, the values of sigma(b) and epsilon(b) of the M sheet in a dry state were 32.4 MPa and 1073%, respectively. Interestingly, the values of ab and 6b of the M sheets prepared from mixing dispersion were larger than those of the G sheets, suggesting that the effect of casein/WPU composite nanoparticles dominated, even over that of the WPU grafted with casein. Moreover, the miscibility of the M sheets was higher than that of the G sheets because of the larger specific surface of the nanoparticles. The particle size and content of casein in the aqueous dispersions containing WPU play an important role in the enhancement of mechanical properties of the composite sheets M and G.