Cellulose is frequently used to strengthen biocomposite films, but few literature systematically deliberates the effects of concentration of celluloses in different geometries on the reinforcement of these composites. Here we prepared three types of celluloses, including rod-like cellulose nanocrystalline (CNC), long-chain cellulose nanofiber (CNF) and microscopic cellulosic fines (CF). The effect of concentration of the three celluloses was examined on the barrier properties to water and light, thermostability, microstructure, and mechanical properties of collagen (COL) films. The addition of celluloses increased the watertightness and thermostability of composite films. Besides, FTIR showed a increased hydrogen bonding for COL/CNF and COL/CNC composite films, but decrease for COL/CF composites. As the concentration of CF and CNF increased, the strength of composites improved. The TS for COL/CNF (124 MPa) and COL/CF composites (113 MPa) were largely increased, compared with that of collagen ones (90 MPa). Considering the factors of crystallinity, hydrogen bonding, and interfacial tortuosity, COL/CNF composites possessed better mechanical behaviors than that of COL/CF and COL/CNC composites. Furthermore, Halpin-Kardos and Ouali models well predicted the modulus of COL/CNF composites when CNF was below and above percolation threshold (2.7 wt%), respectively.