In this study, the biodegradability, morphology, mechanical, and thermal properties of composite materials composed of polybutyleneterephthalate (PBT), acrylic acid-grafted PBT (PBT-g-AA), and sisal fibers (SFs) were evaluated. Composites containing acrylic acid-grafted PBT (PBT-g-AA/SF) exhibited superior mechanical properties because of their greater compatibility with SF than PBT/SF. The dispersion of SF in the PBT-g-AA matrix was highly homogeneous due to ester formation and the creation of branched and cross-linked macromolecules between the carboxyl groups of PBT-g-AA and the hydroxyl groups of SF. Furthermore, due to its lower melting temperature (T (m)), the PBT-g-AA/SF composite was more readily synthesized. Each composite was subjected to biodegradation tests in a soil environment. Both the PBT and PBT-g-AA/SF composite films were completely degraded, with severe disruption of the film structures observed after 60-100 days of incubation. Although the degree of weight loss following burial indicated that both materials were biodegradable, even with high levels of SF loading, the higher water resistance of PBT-g-AA/SF films indicated their higher biodegradability than the PBT films.