The new biodegradable starch graft copolymer, starch-g-polycaprolactone (PCL), was synthesized by using the ring-opening graft polymerization of a epsilon-caprolactone (CL) monomer onto starch backbone. The grafting reactions were conducted with various starch/CL/water feed ratios to obtain starch-g-PCL copolymers with the various PCL graft structures. The detailed microstructure of the starch-g-PCL was characterized with one- and two-dimensional NMR spectroscopy, and the effect of feed composition on the resulting microstructure of the starch-g-PCL was investigated.