In this article, flexible nylon 6,6 was reinforced with rigid-chain aromatic polyamides based on poly(4,4'-diphenylsulfone terephthalamide) (PSA), poly(p-diphenyl oxide terephthalamide) (POA), poly(p-diphenylmethane terephthalamide) (PMA), and isophthaloyl chloride (IPC). Various high molecular weight block copolyamides were synthesized by solution polymerization using p-aminophenylacetic acid (p-APA) as a coupling agent. Their thermal properties show that the block copolyamides exhibit higher values of T-g and T-m and better thermal stability than those of nylon 6,6, especially the IPC-modified nylon 6,6. The order of increased thermal properties of copolyamides is IPC > POA > PMA > PSA. From wide-angle X-ray diffraction patterns, it was found that nylon 6,6 has two diffraction peaks, that is, 2 theta = 20.5 degrees and 23 degrees, while the multiblock copolymers showed only one at 2 theta = 20 degrees, indicating a different crystal structure. It was found that the mechanical properties of the IPC-modified nylon 6,6 were improved more than those of the semirigid copolyamides. The order of tensile strength was IPC > PSA > PMA > POA, but for elongation, it was POA > PMA > PSA > IPC.