In this work, the effects of particle size and content of clam shell powder (CSP) on the mechanical properties and microstructure of clam shell powder-polycaprolactone (CSP-PCL) composites were studied. Accordingly, shell powder with particle size of 50, 100, 200, and 400 meshes, and CSP content of 30, 40, 50, and 60 wt%, respectively, were utilized to prepare CSP-PCL composites. The interface structure and properties of CSP-PCL composites were investigated by infrared spectroscopy (IR), X-Ray diffraction (XRD), scanning electron microscopy (SEM), differential scanning calorimetry, thermogravimetric analysis (TGA), and tensile tests. The results showed that the tensile properties of the composite materials correlated with the particle size and content of the filler. For instance, the smaller the particle size of CSP (400 meshes), the better the tensile properties of the composite. What is more, the tensile properties of the composites were negatively affected by the amount of CSP content. In terms of microstructure, shell powder particles could disperse uniformly in the PCL matrix, and the CSP-PCL composites with polyurethane prepolymer as compatibilizer gave a better compatibility.