The use of graft copolymers of starch and glycidyl methacrylate (starch-g-PGMA) to improve the mechanical properties of composites with poly(hydroxy butyrate-co-valerate) (PHBV) has been investigated. In general, the tensile and flexural strengths of the composites were greater with starch-g-PGMA compared to untreated starch and increased with increasing graft content. The modulus and elongation were not significantly changed by grafting. All samples gained weight after immersion in water for 28 days. Tensile strength and modulus decreased with water sorption, while the fracture toughness significantly increased with grafted starch. No differences were observed between properties of grafts prepared with eerie ammonium nitrate or ferrous sulfate-peroxide graft initiators. Scanning electron micrographs of cryogenic fracture surfaces showed improved adhesion between the starch-g-PGMA and the PHBV matrix. Although no spectroscopic evidence of reaction between PHBV and the starch-g-PGMA was found, the improvement in mechanical properties is consistent with enhanced interactions between the starch-g-PGMA and the PHBV matrix compared to ungrafted starch.