Carbon fiber-reinforced plastics (CFRP) are widely used in the plastics industry. The production of CFRP is mostly carried out on the twin-screw extruder. Many different process variables change the process ability and the final fiber length in the product. In the literature, some models for fiber length decrease along the extruder with glass fibers can be found. The effects of the processing conditions on the final length for glass fiber are determined. In comparison, the incorporation behavior of the carbon fibers are not sufficiently analyzed. The purpose of this paper is to predict the fiber breakage along the screw for carbon fibers. First of all, the buckling case must be determined. The basis of the modeling is a fragmentation equation, for which a break probability and a break rate are defined. A comparison with experimental data shows satisfactory results.