Novel castor oil-based polyurethane/alpha-zirconium phosphate (PU/alpha-ZrP) composite films with different alpha-ZrP loading (0-1.6 wt %) and different NCO/OH molar ratios were synthesized by a solution casting method. The characteristic properties of the PU/alpha-ZrP composite films were examined by Fourier transform infrared spectroscopy (FTIR), thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and tensile testing. The results from Fourier transform infrared spectroscopy indicated that strong intermolecular hydrogen bonding formed between alpha-ZrP and PU, XRD and SEM results revealed that the alpha-ZrP particles were uniformly distributed in the PU matrix at low loading, and obvious aggregation existed at high loading. Because of hydrogen bonding interactions, the maximum values of tensile strength were obtained with 0.6 wt % alpha-ZrP loading and 1.5 of NCO/OH molar ratio in the matrix. Evidence proved that the induced alpha-ZrP used as a new filler material can affect considerably the mechanical and thermal properties of the composites. (c) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 121: 1815-1822, 2011