In this study, the spray dried nanofibrillated cellulose (NFC) was modified by a silane coupling agent to be used in polypropylene (PP)-copolymer matrix in the presence of maleic anhydride grafted PP (PP-g-MAH) compatibilizer. The PP/NFC composites were melt compounded and injection molded. In the first stage, the NFC was characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and atomic force microscopy (AFM). It was shown that fibrillation in aqueous media through high-pressure homogenization was successfully achieved. The nanofibrils having diameter smaller than 50 nm were obtained. The X-ray photo electron spectroscopy (XPS) showed the presence of the silane groups on the NFC surface. Elastic modulus improved by the addition of the high modulus NFC to PP, but the yield strength was not improved. On the other hand, different than many inorganic micron-sized fillers, the elongation-at-break value did not drop much. Differential scanning calorimeter experiments indicated that the NFC acted as a nucleating agent. Rheological investigations exhibited that NFC decreased the viscosity of the PP. The addition of NFC to PP improved the thermal resistance of the PP as can be understood from the TGA experiments. It was demonstrated that renewable nanofiller can be a feasible method to functionalize the existing synthetic polymers. POLYM. ENG. SCI., 2019. (c) 2019 Society of Plastics Engineers