The crystallization behaviors and crystalline morphologies of initial and chain-extended nylon 66 samples were studied with differential scanning calorimetry and polarizing microscopy, respectively. The crystallization rate increased and the size of the spherulites decreased with chain-extension. In addition, a two-peak behavior was observed in chain-extended nylon 66. These results showed that for the chain-extended nylon 66, there was heterogeneous nucleation during crystallization. Therefore, the chain extender possibly functioned as a nucleating agent at the same time. (C) 2003 Wiley Periodicals, Inc.