Water-resistant films were prepared by coating the surface or regenerated cellulose films withcastor oil-based polyurethane (PU) / poly-(methacrylate-co-styrene) [P(MA-St)]. The erfects of the ratioof PU to P(MA-St) copolymer on tensile strength (dry and wet states), vapor permeability, size stability,and water resistivity of the coated films were studied. The interracial interaction between cellulose and thePU/P (MA-St) coating was analyzed using infrared (IR), ultraviolet (UV), scanning electron microscopy(SEM), transmission electron microscopy (TEM), differential thermal analysis (DTA), and electron probemicroanalysis (EPMA). The results indicated that the mechanical properties and water resistlvity of the coated rilms significantly enhanced, and the biodegradability was displayed, when the ratio or PU to P(MA-St)was 8: 2 by weight. The chemical bonds and hydrogen bonds between the cellulose, PU, and the copolymerexist in the coated films. It is regarded that PU/P(MA-St) semi-interpenetrating polymer networks (IPNs)were formed, and a shared network of PU with both the cellulose and the coating in the coated film occurredsimultaneously, resulting in a strong bonding between the coating layer and the film.