A thermoformable starch-graft-polycaprolactone biocomposite was prepared by initiating ring-open polymerization of caprolactone monomer onto starch under microwave irradiation. In this case, the thermoplastic and hydrophobic modification of starch could be realized by one-pot grafting PCL, where the grafted PCL chains acted as the "plasticizing" tails of thermoforming and as the hydrophobic species of water-resistance. The resultant biocomposites were injection-molded as the sheets and their structure and properties were investigated by Fourier transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy, differential scanning calorimetry, dynamic mechanical analysis, contact angle measurement, and tensile testing. In this case, the grafted PCL chains entangled each other, and hence contributed to the strength and elongation of biocomposites. This work provided a simple strategy of one-pot thermoplastic and hydrophobic modification of starch, and may be applied in a Continuous process of modification, compounding, and molding. Meanwhile, the resultant biocomposites containing starch are believed to have a great potential application as an environment-friendly and/or biomedical material. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113: 2973-2979,2009