The highly versatile fused filament fabrication (FFF) allows on-demand manufacturing of carbon fiber reinforced plastics (CFRP) structures with intricate geometries. However, the intrinsic low throughput and comparatively low strength of FFF have posed major hurdles for large-scale industrial applications. In this work, high strength PA6-CF composite parts containing up to 35 wt% CF are additively manufactured by directly feeding the composite pellets into a custom-designed screw-extrusion 3D printer. The effects of CF content on the rheological and mechanical properties of the PA6-CF composites are investigated in detail. Mechanical tests show that CF significantly enhances the strength of the 3D-printed composites. With 25 wt% CF, the tensile strength reaches maximum at 169.7 MPa, which is 3.2 times higher than the unfilled PA6, and is also among the highest for the 3D printed CFRPs reinforced by short fibers. In-depth microstructural characterization reveals the printed part retains long carbon fibers (average length over 200 μm) and has comparatively low porosity (2.93% at 25 wt% CF), which are conducive to achieve high mechanical strength. The mechanical strength also agrees reasonably well with the analytical model based on Kelly-Tyson equation. Display omitted •Successfully prepared PA6-CF composite pellets with three CF loadings (up to 35 wt%).•3D printing the pelletized composites with a screw-extrusion 3D printer.•The printed PA6-CF composites exhibit very high tensile strength, reaching 170 MPa.•The reinforcement saturation is identified around 25 wt% CF content.