Nowadays, environmental concerns are leading toward investigations on more and more environmentally friendly nanocomposites. However, those materials must be as efficient as existing nanocomposites. Cellulose filaments (CF) are unique natural fibers, with high aspect ratio and high surface area obtained by mechanical peeling of softwood pulp fibers. Each fiber can be peeled into about 1000 CFs, with lengths between 100 and 2000 µm and widths between 30 and 500 nm. It was shown in some publications that their addition to polymers, at relatively low loading levels, can result in enhanced mechanical properties. However, producing those nanocomposites by melt mixing remains a challenge because of the strong bonding network in between the fibers, which makes it challenging to obtain a good dispersion. In this work, polypropylene (PP) reinforced with CF nanocomposites were studied. Nanocomposites with CF loadings ranging from 0 to 30 wt% were produced by melt extrusion followed by injection molding. Rheology and optical observations showed that the dispersion state of CF was satisfying. However, tensile and impact tests showed that cellulose filaments had a limited effect on the mechanical properties of PP. Consequently, two leads were investigated: • The effect of CF on the crystalline structure of PP was studied using differential scanning calorimetry, small angle X-ray scattering and wide X–ray scattering. The results showed that the crystallization temperature increased with addition of CF. • The effect of addition of a compatibilizer to improve interfacial adhesion between PP and CF was studied as well. The presence of a compatiblizer led to an enhancement of mechanical properties of CF nanocomposites. Overall, this work shows that a good dispersion besides a good adhesion between the components is necessary for the fibers to enhance the mechanical properties. In the case of PP reinforced by cellulose filaments, the addition of a compatibilizer was necessary.