The fiber degradation in the kneading disk region of a corotating twin screw extruder was studied with a closed single-stage co-rotating twin screw extruder as a model for the analysis. In a previous article, we reported that the stress in the kneading zone was proportional to the screw torque. In the present study, we measured the fiber-length distributions and the average fiber lengths under various compounding conditions. It was found that the fiber-length distributions depend upon the shear stress and the total number of rotations and that the fiber-length distributions were identical if the average fiber lengths were the same.