The capillary extrusion behaviors of high-density polyethylene (PE) containing carbon fibers (CF) were investigated from a viewpoint of their rheological properties. For values of CF content ranging from 0 to 5 wt%, the oscillatory moduli of PE slightly increased with CF content. Transient shear flow measurements showed that the steady shear viscosity and first normal stress difference of PE also slightly increased with the addition of CF. Conversely, the strain hardening behaviors of PE were suppressed by CF addition. It was found that CF becomes well dispersed in the PE matrix and that the elongation of the PE chain can be prevented by the presence of CF. The swell ratio of PE, which is the ratio of the diameters of the extrudate and the die, decreased with CF. Using a wide entrance angle die, PE/CF exhibited helical distortion at a higher output rate than that of PE. Using a narrow entrance angle die, sharkskin failure was observed in PE/CF at a higher output rate than in PE, although the helical distortion of PE was inhibited by CF. We discuss the origin of the helical distortion as an effect of CF on the swell behavior and the flow instabilities of PE in reference to the rheological behavior.