The effect of the surface roughness of carbon fibers (CFs) on interfacial adhesion was investigated by comparing polyamide­(PA)­6-based composites using a commercial CF (T700) and a low-cost textile polyacrylonitrile-based CF (TexCF). The tensile strength of T700CFs was 66% higher than that of TexCFs; however, the Izod impact, interlaminar shear, and flexural strength of the long-fiber-reinforced thermoplastic (LFT) composites fabricated using the two CFs were significantly similar. The TexCFs with a rougher surface had a higher wettability than that of T700CFs, resulting in a higher interfacial shear strength value of TexCF/PA6 composites despite the much lower tensile strength of TexCFs than that of T700CFs. This suggests that the high surface roughness of CFs plays a critical role in strengthening the interfacial adhesion between the CFs and the matrix. It is concluded that low-cost CFs with high roughness are promising reinforcement candidates for the fabrication of adequate LFT composites despite their low tensile strength.