Carbon fiber (CF)-embedded acrylonitrile butadiene styrene polymer composites printed on the large-scale printer at Oak Ridge National Laboratory were investigated by small-angle neutron scattering to correlate the microstructure of the composites with their mechanical strength. The microstructure of the polymer domains and the alignment of CF were characterized across the interfaces between layers of the hot-melt extruded material and were compared with CF-free ABS. The small-angle neutron scattering data show that the CF-containing material displays strong anisotropic scatterings suggesting molecular alignment along the printing direction that is not present in the CF-free ABS. Scattering data analysis across the interfacial layer revealed enhanced molecular alignment along the printing direction near the boundaries and inhomogeneous size distribution of polymer domains upon the addition of CF. We attribute the compromised strength across interfacial layers from CF-containing material to this inhomogeneous size distribution which prevents effective lateral interaction between layers. POLYM. ENG. SCI., 59:E65-E70, 2019. (c) 2018 Society of Plastics Engineers