ABSTRACT Research into bio filler materials composite is gradually taking prominence due to its renewability. In this present work, three particle sizes (400µm, 600µm and 800µm) of milled periwinkle-shell (PWS) filled cashew nut shell liquid (CNSL) resin composites were developed by compression moldings technique with varying filler content (10% to 40% by weight). The developed PWS filled CNSL composites were then characterized by tensile, flexural and impact strength tests. The effect of filler content and particle size on the mechanical properties of the PWS filled CNSL composites was investigated. Results showed that, there was an improvement on the mechanical properties as the filler content increases while properties decreases as filler size increases. All properties tensile strength, flexural strength, % elongation, tensile modulus and impact strength slightly increased as the filler loading increases. While % elongation, tensile and flexural strengths decreases as particle sizes increases, tensile modulus and impact strength increases as particle sizes increases. The optimum properties were observed at 30% filler content and 400µm particle size