The effect of screw rotation speed on the mechanical property and thermal conductivity of polycarbonate (PC)/vapor-grown carbon fiber (VGCF) composites prepared by a twin screw extruder was discussed in this paper. Two types of VGCF (VGCF-H, the aspect ratio of 40, and VGCF-S, the aspect ratio of 100) were used. In the tensile test, the breaking pattern of PC composite changed to brittle failures by adding VGCF-H irrespective of screw rotation speed. Young's modulus of PC/VGCF-H slightly increased with the screw rotation speed. On the other hands, the breaking strain of PC/VGCF drastically decreased above 150 min(-1). Young's modulus of PC/VGCF-S slightly increased with screw rotation speed until 150 min(-1), however, it decreased at 175 min(-1). The thermal conductivity of PC/VGCF-H was independent of the screw rotation speed. In contrast, the thermal conductivity of PC/VGCF-S gradually increased with screw rotation speed until 150 min(-1) and it also decreased at 175 min(-1). From the SEM observation and rheological behavior, the dispersion state of VGCF-H in PC/VGCF-H was independent of the screw rotation speed. On the other hands, the network structures of VGCF-S were observed in PC/VGCF-S and the state of these network structures depended on the screw rotation speed. It was clarified that the mechanical property and thermal conductivity of PC/VGCF were attributable to the dispersion state of VGCF.