The biodegradability, morphology, and mechanical thermal properties of composite materials composed of polylactide (PLA) and sisal fibers (SFs) were evaluated. Composites containing acrylic acid-grafted PLA (PLA-g-AA/SF) exhibited noticeably superior mechanical properties because of greater compatibility between the two components. The dispersion of SF in the PLA-g-AA matrix was highly homogeneous as a result of ester formation and the consequent creation of branched and crosslinked macromolecules between the carboxyl groups of PLA-g-AA and hydroxyl groups in SF. Furthermore, with a lower melt temperature, the PLA-g-AA/SF composite is more readily processed than PLA/SF. Both composites were buried in soil to assess biodegradability. Both the PLA and the PLA-g-AA/SF composite films were eventually completely degraded, and severe disruption of film structure was observed after 6-10 weeks of incubation. Although the degree of weight loss after burial indicated that both materials were biodegradable even with high levels of SF, the higher water resistance of PLA-g-AA/SF films indicates that they were more biodegradable than those made of PLA. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 123: 347-355, 2012