The thermal and mechanical properties of castor oil/polycaprolactone-based polyurethane (CPU) films and polyurethane biocomposites reinforced with hemp fibers (HCPU) were investigated. Although similar films can be synthesized from petroleum, the main interest in studying these biomass-based composites is based on the fact that both fiber and matrix are derived from renewable resources. In this study, castor oil was used as a polyol for polyurethane films and hemp fiber was used to reinforce the biocomposites. To control the mechanical properties of CPU and HCPU, polycaprolactone diol (PCL) was added to the polyol mixture. Varying the mixing ratio of castor oil and PCL, the thermal and mechanical properties of the CPU and HCPU samples were investigated by using FT-IR, DSC, DMTA, Minimat, and SEM. In an attempt to improve interfacial adhesion between the fiber and matrix biocomposites, hemp fiber was reacted with MDI. FE-SEM micrographs showed that the surface of the hemp fiber became smoother after reaction with MDI. Urethane bonding formation was confirmed by FT-IR.