Thermoplastic nanocomposites were industrially developed to enhance materials properties by adding low amounts of fillers to it. Environmental concerns are nowadays leading toward investigations on novel nanocomposites which are more environmentally friendly but as efficient as glass fibers and carbon fibers nanocomposites to develop new materials for construction or automotive components. Natural fibers are of interest because of their low cost, low density, high specific strength and modulus, renewability and biodegradability. Cellulose filaments (CF), which can be mechanically obtained from cellulose microfibrils, are semi-crystalline and biodegradable nanofibers. It was shown in few publications that their addition to polymers, at relatively low loading levels, can result in an improvement of the mechanical properties of the material while maintaining good flow properties and low density which makes them suitable candidates for new eco-friendly nanocomposites. However, those nanocomposites were almost all produced by solution casting because, even if melt mixing is the most promising technique for industrialization, the highly entangled structure of the fibers makes it quite challenging to disperse correctly the CFs using direct melt mixing. 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 and characterized. Rheology and optical observations were used to estimate the dispersion state of CFs. Tensile tests were conducted to study the mechanical behavior of the materials. Results showed that nanocomposites with improved mechanical properties (higher rigidity and higher strength), comparable to the properties of a glass fiber nanocomposites, can be obtained when a suitable dispersion is achieved.