In the olive oil-producing countries, large quantities of olive solid waste are dumped in nature causing several environmental problems. Therefore, the use of this by-product as filler in (ethylene-propylene) matrix could be an effective way to reduce significantly the quantities of the disposed biomass and elaborate a cost-effective material. However, the hydrophilic nature of this natural filler decreases the compatibility with the hydrophobic matrix. To improve the interfacial adhesion, maleated polypropylene was added as a compatibilizing agent. In a first step, the influence of the filler loading on the rheological, thermal, and mechanical properties was investigated. Composites with various filler contents were prepared by melt blending using a twin-screw extruder and injection molding. The filler addition led to an increase of the viscoelastic properties and the crystallinity degree. Then, the coupling agent (MAPP) was reported to enhance the rheological, thermal, and mechanical properties of the composites up to a critical amount beyond which the plasticizing effect becomes more predominant than the reinforcing effect, a fact that could be responsible for the decrease of viscoelastic properties, crystallinity, and mechanical properties. POLYM. ENG. SCI., 56:27-35, 2016. (c) 2015 Society of Plastics Engineers