Rheological and mechanical properties of the polycarbonate (PC)/carbon fiber (CF) composites with different initial fiber length, sizing effect of the fiber, fiber content and screw speed have been studied. PC/CF composites were prepared by twin screw extruder. In the studies of sizing effect of the fiber on the rheological properties of the PC/CF composites, the viscosities of the composites prepared with the sized carbon fibers gave higher values than that prepared with unsized carbon fibers because of the increase in the fiber length after extrusion for the sized fibers. From the results of the effect of screw speed on the rheological and mechanical properties of PC/CF composites, we found that the viscosity and tensile strength decreased with increasing screw speed. This is mainly due to that the fact that the fiber length after extrusion has been decreased from 163 to 148 pin with the increase of the screw speed from 100 to 200 rpm when the fiber content used in the composites is equal. From the above results, it is suggested that the fiber length after extrusion of the PC/CF composites has a strong effect on the rheological and mechanical properties of the composites. Also, a relationship among the initial fiber length, sizing effect of the fiber, fiber content and screw speed has been studied comprehensively for the PC/CF composites. (C) Koninklijke Brill NV, Leiden, 2009