Composite films consisting of two biocompatible natural polymers were prepared by incorporating bacterial cellulose into silk fibroin. The high crystalline cellulose hydrogel was synthesized by bacteria, Acetobacter xylinum. Silk fibroin was finely blended with the bacterial cellulose fibrils by adding aqueous silk fibroin solution into the bacterial cellulose hydrogel. We observed individual bacterial cellulose fibrils within a silk fibroin matrix by field emission scanning electron microscopy. The silk fibroin well penetrated between the individual fibrils of the bacterial cellulose. The silk fibroin/bacterial cellulose composite film did not dissolve in water without further crystallization process, whereas a pure silk fibroin film easily dissolved in water without crystallization process. The crystallization phenomenon of silk fibroin in the composite film was verified by X-ray diffraction and Fourier transform-infrared spectroscopy. Furthermore, the mechanical properties of the silk fibroin/bacterial cellulose composite film markedly increased when the composite film was hydrated. The silk fibroin/bacterial cellulose composite films became more flexible and tougher when they were hydrated, while it was very brittle in the dehydrated state.