This study presents the experimental results of glass fiber attrition in molten polypropylene in a co-rotating twin screw extruder. The number average length decreased with increase of screw speed as well as glass fiber mass per resin. The set of ordinal differential equations of the Phelpes model was integrated in the numerical simulator of twin screw extruder based on the Hele-Shaw flow recently developed by our group. The number-average glass fiber length of experiments and simulation agreed well each other.