Chemical modification of cellulose fibers with a trialkoxysilane has been performed and modified cellulosecastor oil polyurethane composites have been successfully studied. The modified cellulose and all composites was characterized by Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), thermogravimetry (TGA) and water uptake test (WU). Compared with the crude cellulose fibers, the organosilane treatment reduces the hydrophilicity of the natural fibers and increases the fiber/matrix compatibility. In addition, the better dispersion of treated cellulose fibers was characterized by scanning electron microscopy (SEM). Cellulose fiber modification effect on mechanical properties was investigated by uniaxial traction test. Results show an improvement of Young modulus and tensile strength. Composites thermal properties of are also improved after the organosilane treatment of fibers. These results are due to the interfacial adhesion improvement by the formation of chemical bonding between prepolymer isocyanate extremities and the glycidol grafted on cellulose surface.