This work incorporated hexamethylene diamine and terephthalic acid, namely 6T, into typical polymerization processes of Nylon 6 and Nylon 66 to form Nylon 6/6T and Nylon 66/6T copolymers, respectively. The WAXD results showed that as the amount of 6T increased to 10 mol%, all of the Nylon 6/6T copolymers transferred from alpha-form to gamma-form. The DSC measurement revealed that the equilibrium melting point of Nylon 6/6T decreased as the amount of 6T increased. Moreover, the crystallization rate k decreased with the 6T content in Nylon 6/6T copolymers, and dropped greatly when the addition of 6T increased over 7 mol%. In contrast, due to the similar molecular chain lengths of Nylon 66 and 6T, the WAXD results of Nylon 66/6T exhibited little crystal difference as the addition of 6T varied from 0 to 30 mol%. The equilibrium melting point of Nylon 66/6T remained constant, proving that Nylon 66/6T is isomorphous. (C) 2013 Elsevier B.V. All rights reserved.