A D-optimization design was applied to investigate the strength properties of biomaterial-doped starch films. The individual effect of each additive and associated interactive effects were investigated. Cellulosic fibers and chitosan were able to substantially enhance film strength. Lignin and gelatin also demonstrated significant interactive effects on strength, while different interactive effects were evident between the major biopolymers (fiber and chitosan) and minor biopolymers (lignin and gelatin). The film stretch depended on the material composition where diminishing returns were observed for stretch, while strength increased for increasing levels of each of the biopolymers. An increase in fiber and chitosan contents led to increases in film density that correlated well with strength. The latter data indicate that fiber and chitosan biopolymers act synergistically by providing increased film bonding that eventually generates a stronger film.