In composites of hydroxyapatite (HA) nanoparticles with a polymer matrix, the aggregation of nanoparticles would induce structural defects. In order to improve the dispersibility of HA nanoparticles in poly(ethylene glycol) dimethacrylate (PEGDMA) matrix and enhance mechanical properties of the HA/PEGDMA composite as potential bone substitute material, surface-grafted HA nanoparticles with poly(ethylene glycol) monomethacrylate (PEGMA) were prepared, and crosslinked with PEGDMA under UV light to form a composite. The structure of HA-g-PEGMA was characterized by X-ray diffraction (XRD) and thermal gravimetric analysis (TGA). The dispersibility of HA-g-PEGMA nanoparticles in poly(PEGDMA) was evaluated by SEM. The mechanical properties of the composites were investigated by compressive test. The dispersibility of HA-g-PEGMA nanoparticles in poly(PEGDMA) matrix was better than the bare HA. At a 1 wt % content of loading, the strength of composites increased by 14%, and the modulus increased by 9%. (c) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013