Poor physical properties resulting from low interfacial interactions between hydrophilic biopolymers and hydrophobic thermoplastic matrices have been one of the biggest obstacles in preparing quality biomass materials. This study concentrates on the effects of nano-TiO2 on the properties and structure of starch/poly (epsilon-caprolactone) (PCL) composites. The molecular and crystal structures of the composites were characterized by using Fourier transform infrared spectroscopy, differential scanning calorimeter (DSC), X-ray diffraction (XRD), and field emission scanning electron microscope. The results indicated that an interpenetrating network structure formed by adding nano-TiO2 into starch/PCL composites. The DSC and XRD analysis indicated that the crystallinity degree and the crystallization rate of the composites reduced, whereas the crystal form and crystal size were unchanged. The results also showed that the mechanical properties and water resistance of the composites were improved significantly with the addition of nano-TiO2, whereas their transparency decreased. (c) 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 4129-4136, 2013