Abstract: An inline compounding technique by modifying single screw extruder was proposed for manufacturing long glass fiber reinforced polypropylene thermoplastic composites. The effects of hole diameter, open hole ratio, hole pitch of breaker plate and distance between screw tip and breaker plate on the morphology of long glass fiber were studied, and the mechanical properties of the composites were tested. The results show that the breaker plate has an obvious impact while the change of hole pitch has not on the morphology of long glass fiber when the ratio of hole diameter to fiber length is below 0.5 and the open hole ratio less than 0.2. Compared to the single hole one, breaker plate with multi holes is favorable for fiber distribution and further improvement of the mechanical properties of the composites. A certain distance between the breaker plate and screw tip should be kept to maintain fiber length and stable extrusion. For a given breaker plate, the local resistance coefficient is found to be a reliable parameter to control the fiber morphology.