A new method of the surface modified starch (SM-St) by grafting reaction of the hydroxyl groups on the starch with L-lactic acid was developed. A novel biodegradable starch grafted copolymer, starch-g-poly(L-lactide) (St-g-PLLA), was synthesized in situ by the ring-opening graft polymerization of a L-lactide (LLA) monomer onto the surface of the SM-St granules in the presence of Sn(Oct)(2) as a catalyst. The PLLA grafting efficiency could reach as high as 64 wt%. The structure of St-g-PLLA was characterized by IR, DSC and WAXD. The good adhesion between the two components had been evidenced by SEM observations. The medium-resistance of St-g-PLLA and the mechanical properties of the PLLA composite blending with St-g-PLLA were much better than that of the PLLA/starch blending composite. (c) 2005 Elsevier Ltd. All rights reserved.