We propose a composite 3D printing method using an ultralow-melt-viscosity polymer to achieve a low void content and high strength compared to that achieved using conventional viscosity polymer. Using continuous carbon fibers as the core of the filament, ultralow-viscosity polymers, which could not be filamentized by polymer alone, could be used. In the present study, polyether ether ketone/carbon fiber (CF/PEEK) filament extrusion was performed and the effect of the melt viscosity on the voids in the filament was evaluated. The ultralow-viscosity polymer (CF/PEEK 90 G) reduced the voids in the filaments by 92% compared to the standard polymer (CF/PEEK 450 G). The interlaminar tensile strengths of 3D-printed CF/PEEK filaments fabricated with different melt viscosities of the PEEK polymer were also evaluated. The interlaminar tensile strengths of CF/PEEK 90 G was improved by 116.8% compared to that of CF/PEEK 450 G.