A series of 10T/10X (X = 6, 8, 10, 12, 14, 16) copolyamides was synthesized, and the thermal and mechanical properties were investigated. The effects of the lengths of alkyl chains of diacids on their resultant properties were studied, and we found that the introduction of a small amount of aliphatic diacid hardly changed the lattice parameters of the copolyamides. The T-m and T-c values of the 10T/10X copolyamides decreased and had a zigzag oscillation behavior as the lengths of alkyl chains on the diacids increased. A density functional theory study showed that the observed behaviors were related to the change in hydrogen bond strength and the length mismatches of the diacids in the copolymer chains. The tensile strength decreased slightly, while the elongation at break increased significantly with an increase in the chain length of the aliphatic diacid; further, the notched impact strength decreased but was still higher than PA10T.