AbstractThe effects of the amounts of flour from the red pepper (Capsicum annuum) fruit stem (RPFS), together with coupling agent (CA), on the mechanical and physical properties of polypropylene (PP)-based composites were investigated. Pellets manufactured through single screw extruders were injection molded into composite samples. Density, mechanical property, and dimensional stability of manufactured composites were determined according to ASTM standards. Results were analyzed using central composite design (CCD). Statistical analyses showed that filler loading significantly affected the density, as well as mechanical and physical properties of thermoplastic composites. Density of the composites was increased with filler loading but not affected by coupling agent amounts. In the case of mechanical properties, tensile modulus, flexural strength, and flexural modulus were improved with increasing filler loading while the tensile strengths, elongation at break, and impact strength of the samples were decreased. The tensile strength of the thermoplastic composites was positively affected by CA contents, but other mechanical properties were not affected as much. In the case of physical properties, thickness swelling and water absorption of the composites were increased with increasing weight percent of RPFS flour. However, these properties were not significantly changed by CA addition. Overall results revealed that RPFS flour could be potentially suitable raw materials for thermoplastic composites.