Castor oil based polyurethane (PU) interpenetrating polymer network (IPN) has been prepared by using castor oil, toluene diisocyanate and styrene monomer. The coconut shell powder (CSP) particulate filler with different weight ratios viz., 0, 5, 10, 20 and 30 wt % were incorporated into polyurethane/polystyrene (PU/PS, 90/10) IPN. The effects of CSP particulate filler loading on the physico - mechanical and thermal properties of PU/PS IPN biocomposites have been studied. From the tensile behavior, it was noticed that a significant improvement in tensile strength and tensile modulus with increase in CSP filler content. It was found that the degradation of all composites takes place in two steps. Degradation kinetic parameters were obtained for the composites using Broido (BR), Coats–Redfern (CR) and Horowitz-Metzger methods (HM). The damping property reduced after the incorporation of CSP filler. Morphology of the cryo-fractured composites was studied using scanning electron microscopic technique (SEM).