Poly(acrylic acid)-g-polycaprolactone (PAA-g-PCL) graft copolymer was synthesized and starch/PCL blends compatibilized with PAA-g-PCL were prepared. The mechanical properties of the starch/PCL blends compatibilized with various PAA-g-PCLs that have different graft degrees and graft lengths were investigated. As the graft degree of the PAA-g-PCLs that have the same graft length increased, the modulus and the strength of the blends decreased. However, the elongation at break and the tensile toughness of the blends showed a maximum at a certain graft degree (10.8 mol%) owing to the better compatibilizing effect compared to the low (3.9 mol%) and the high (23.4 mol%) graft degree of PAA-g-PCL. It was also found that the modulus and the strength of the blends increased with the increase of graft length of the PAA-g-PCLs that have the same graft degree (similar to 11 mol%). However, the blend compatibilized with the short graft length (M. W. of PCL graft: 530) exhibited the highest value of the elongation at break and the tensile toughness. This result is attributed to the self-crystallization of PAA-g-PCL in the blend that has longer PCL grafts.