The aim of this study was to improve the mechanical properties of thermoplastic starch foams prepared from cassava starch blended with natural rubber latex by reactive blending. Potassium persulfate was used as an initiator for graft copolymerization between the starch and natural rubber during baking. The starch-natural rubber graft copolymer (starch-g-NR copolymer) was successfully produced during both Suspension and melt blending based on H-1-NMR and FTIR characterization. Natural rubber increased the flexural modulus of starch/natural rubber foams without potassium persulfate, thus indicating the compatibility of the blends. The starch-g-NR copolymer, acting as a compatibilizing agent, enhanced the impact strength of foams, but it did not improve the flexural modulus. This may be due to the potassium persulfate decreasing the molecular weight of the natural rubber. Relative humidity also played an important role on the mechanical properties. Foams became more ductile at higher relative humidities. Since foam density increased with an increasing natural rubber content, the specific impact strength was also considered. A soil burial test showed that the cassava starch foams and foams containing 15 pph of natural rubber were fully biodegraded within 8 and 18 weeks, respectively. The starch-g-NR copolymer delayed biodegradation of foams and foams containing high natural rubber content, i.e., 35 pph, showed a low ability to be biodegraded. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 116: 93-105, 2010