Novel water-resistant cellulose films from castor-oil-based polyurethane (PU) and benzoyl konjac glucomannan (BKGM) semi-interpenetrating polymer networks (semi-IPNs) forming a superthin coating on the cellulose films, which was regenerated from cellulose in 6 wt % NaOH/5 wt % thiourea aqueous solution, were prepared. The tensile strength (sigma(b)), water resistivity (R), light transmittance (T-r), and size constriction (S-c) of the coated films with coatings of 0.2-0.6 mum thickness and with different BKGM contents in the coatings were investigated. The values of sigma(b), R, and S, were 93 MPa, 0.57, and 0.8%, respectively, when the BKGM content in the coating was 5 wt %. Even when the coating layer thickness was 0.2 mum, the coated films still had higher tensile strength (80 MPa) and water resistivity (0.47) than cellulose film. The results from Fourier transform infrared spectroscopy, ultraviolet spectroscopy, transmission electron microscopy, and electron probe microanalysis indicate that much of the improvement of the mechanical properties can be attributed to the strong interfacial bonding between the coating and the cellulose in the coated films, which was caused by covalent and hydrogen bonding to form shared PU networks in the interface. The BKGM in the coating plays an important role in enhancing the mechanical properties and water resistivity of the coated films because of the presence of the semi-IPN coatings formed by the incorporation of BKGM. The coated films have good size stability and appropriate evaporative capacity, thus giving them promise for applications in the food, medical, and agriculture fields.