We prepared composite films consisting of two biocompatible materials, bacterial cellulose and silk fibroin. Aqueous silk fibroin solution and bacterial cellulose excreted by Acetobacter xylinum were used to fabricate the composite films. It was verified by field emission scanning electron microscopy and X-ray diffraction that the two components were finely blended and that the silk fibroin was crystallized during the composition of the films. The silk fibroin penetrated well between the individual fibrils of the bacterial cellulose, while the water molecules inside the pellicular bacterial cellulose were evaporating. The composite films did not dissolve in water due to the crystallization of the silk fibroin in the composite films. We also observed the change in the mechanical properties of the composite films according to the water content. The composite films became more flexible and tougher when they were dipped in water, whereas they were very brittle in the dehydrated state.