The effects of filler content and chemical treatment on mechanical and thermal properties of polypropylene-chitosan composites were investigated. Filler chemical treatment was carried out by reacting chitosan with organosolv lignin/sodium dodecyl sulphate binary modifying agents under mild condition. Filler-matrix interfacial adhesion enhanced considerably in presence of the binary agent that effectively modified the chitosan surface, leading to increased tensile and impact strength of the composites. Even though the chemical treatments did not change the thermal degradation mechanism of the composites, the obtained results exhibited that the treated composites had better thermal properties than the untreated composites. These finding implies that organosolv lignin-containing binary modifying agents could be a potential reagent to replace synthetic modifying agents.