Semi-interpenetrating polymer network (semi-IPN) coatings were prepared by using castor oil-based polyurethane (PU) and nitrocellulose (NC) with various viscosity-average molecular weights (M ) from 6 Ο« 10 4 to 42 Ο« 10 4 , and coated on a regenerated cellulose (RC) film to obtain water-resistant film. The PU/NC coatings and coated films, which were cured at 80Β°C for 5 min and 2 min, respectively, were investigated by infrared (IR) and ultraviolet (UV) spectroscopy, X-ray diffraction, swelling test, strength test, dynamic mechanical thermal analysis (DMTA), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). The results show that the crosslink densities of the PU/NC semi-IPNs were smaller than that of pure PU, and decreased with the decrease of M of nitrocellulose (NC M ), indicating NC molecules cohered intimately with PU, and hindered the PU network formation. The physical and mechanical properties of the films coated with PU/NC coatings were significantly improved. With the increase of NC M , the strength and thermal stability of the coated films increased, but the pliability, water resistivity, and optical transmission decreased slowly. The PU/NC coating with low NC M more readily penetrated into the RC film, and reacted with cellulose, resulting in a strong interfacial bonding and dense surface caused by intimate blend of PU/NC in the coated films.