The goal of this study was to examine how the extrusion process affects the dispersion, length and thermal degradation of cellulose fibers (CF) in polypropylene (PP) composites. Bleached sulfite cellulose fibers were pelletized and then compounded (20 and 30 wt%) in a co-rotating twin screw extruder. The pelletizing process of cellulose fibers was found to cause extensive fiber breakage. The fiber length was decreased from an initial 1.7 to 0.8 mm. The results showed that higher screw speed, higher fiber content and two extrusions increased the shear energy, resulting in improved fiber dispersion but also caused fiber breakage and thermal degradation. The composites mechanical properties were not improved with better fiber dispersion and the reason for this might be the extensive fiber breakage and thermal degradation. Moreover, it was found that the PP polymer was also slightly degraded during the compounding process. ; The goal of this study was to examine how the extrusion process affects the dispersion, length and thermal degradation of cellulose fibers (CF) in polypropylene (PP) composites. Bleached sulfite cellulose fibers were pelletized and then compounded (20 and 30 wt%) in a co-rotating twin screw extruder. The pelletizing process of cellulose fibers was found to cause extensive fiber breakage. The fiber length was decreased from an initial 1.7 to 0.8 mm. The results showed that higher screw speed, higher fiber content and two extrusions increased the shear energy, resulting in improved fiber dispersion but also caused fiber breakage and thermal degradation. The composites mechanical properties were not improved with better fiber dispersion and the reason for this might be the extensive fiber breakage and thermal degradation. Moreover, it was found that the PP polymer was also slightly degraded during the compounding process.