Sisal whiskers (SW) were prepared by acid hydrolysis for subsequent evaluation as reinforcing material for biodegradable matrices of thermoplastic starch (TPS) and TPS/polycaprolactone (TPS/PCL) blends. The acid hydrolyzed SW had dimensions of 5 ± 2 nm in diameter and 210 ± 60 nm in length and 78% crystallinity. The nanocomposite blends consisted of whisker contents of 5 and 10 wt% and were coextruded with either TPS or TPS/PCL which formed the matrix material. The influence of SW on the morphology, crystallinity, thermal properties, mechanical behavior, structural, and biodegradation of TPS and TPS/PCL matrices were investigated. The lower SW content of 5 wt% dispersed and interacted well with both matrices. The introduction of PCL in the TPS matrix leads to an increased crystallinity and there was evidence that the carbonyl groups of PCL interacted with the OH groups on the surface of the whiskers. Addition of PCL decreased the whisker/whisker interaction over TPS alone, favoring the dispersion of the whiskers in the matrix and, consequently, the reinforcement effect was more pronounced in TPS/PCL blends than in the pure TPS matrix. The addition of the whiskers as reinforcement increased the biodegradability of TPS and TPS/PCL matrices. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers.