A new method of surface modification of hydroxyapatite nanoparticles (n-HA) by surface grafting reaction Of L-lactic acid and ring-opening polymerization Of L-lactide (LLA) was developed. Two modified HA nanoparticles were obtained: HA modified by L-lactic acid (I-HA) and HA grafting with poly((L)-lactide) (PLLA; p-HA). The modified surface of n-HA was attested by Fourier transformation infrared, (31)p MAS NMR, and thermal gravimetric analysis. The results showed that L-lactic acid could be easily grafted onto the n-HA surface by forming a Ca carboxylate bond and initiated by the hydroxyl group of the grafted L-lactic acid and LLA could be graft-polymerized onto the n-HA surface in the presence of stannous octanoate. The highest grafting amounts of (L)-lactic acid and PLLA were about 33 and 22 wt %, respectively. The modified HA/PLLA composites showed good mechanical properties and uniform microstructure. The tensile strength and modulus of the p-HA/PLLA composite containing 15 wt % of p-HA were 67 MPa and 2.1 GPa, respectively, while those of the n-HA/PLLA composites were 45 MPa and 1.7 GPa, respectively. The elongation at the break of the l-HA/PLLA composite containing 15 wt % I-HA could reach 44%, in comparison with 6.5% of the n-HA/PLLA composites containing 15 wt % n-HA.