Thermoformable bionanocomposites of chitin whisker-graft-polycaprolactone (CHW-g-PCL) were synthesized by initiating the ring-opening polymerization of caprolactone monomer onto the CHW surface under microwave radiation. In this case, the "graft from" strategy contributed to long and dense "plasticizing" PCL tails onto the CHW surface as the key of thermoforming, and, therefore, Such bionanocomposites were injection-molded as the sheets with a Structure of cocontinuous phase mediated with the entanglement of grafted PCL chains. The structure and properties of the molded CHW-g-PCL sheets were investigated by FTIR, XRD, SEM, DSC, DMTA, contact angle measurement, and tensile test. With an increase of the PCL content in CHW-g-PCL, the strength and elongation as well as the hydrophobicity of the nanocomposites increased at one time. This is the first report on the thermoformable polymer-grafted nanocrystal derived from natural polysaccharide. Moreover, such new bionanocomposites with good mechanical performances could have great potential applications. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 112: 28302837, 2009