Eco-composites made from poly(lactic acid) (PLA), poly(butylene succinate) (PBS) and corn starch (CS) with a designable interface were prepared by reactive processing using aliphatic isocyanates as coupling agents. Hexamethylene diisocyanate (HDI) and lysine diisocyanate (LDI) were used as aliphatic isocyanates. Copolymers of biodegradable polymers and CS with urethane linkages were produced during the kneading process, as confirmed by NMR spectroscopy. With the addition of isocyanates the tensile properties improved. The interfacial morphology showed that the interfacial adhesion between the polymer matrices and CS was enhanced by the addition of isocyanates. Diffusion coefficients of water were smaller for all composites with isocyanates than without, indicating that the diffusion of water into the composites with isocyanates proceeds slowly. The water absorption rate increased as the CS content rose and decreased in the composites with isocyanates.