Composites based on thermoplastic cornstarch (TPS) and polycaprolactone (PCL) were reinforced with 5, 10 and 20% (wt%) of pretreated sisal fiber. The impact of the addition of sisal fiber on the mechanical, thermal and morphological properties of composites was investigated. The addition of 5-10% fibers in composites exhibited improved mechanical and thermal properties attributed to more efficient dispersibility of fiber in the matrix and good compatibility between fibers and the matrix polymers, which also led to increased crystallinity and caused composite to be more rigid. The DSC and X-ray diffraction studies suggested interaction between polymers in the blend via carboxyl groups in thermoplastic starch-PCL and hydroxyl groups in fibers. An increase in storage energy was also observed from the DMTA studies, indicating mobility reduction in the polymer chains. Crystallization of PCL was apparent in the TPS/PCL blends but not in TPS/PCL composites with the added fibers, suggesting that sisal fibers hindered the crystallization of PCL in composites.