Nylon 11/nylon 66 alloys were prepared by in situ polymerization. Analysis of the isothermal crystallization behaviors of nylon 11/nylon 66 alloys was carried out using differential scanning calorimetry (DSC) and X-ray diffraction (XRD). The crystallization kinetics of the primary stage under isothermal conditions could be described by the Avrami equation. The crystal morphology observed by means of polarized optical microscope (POM). In the DSC scan after isothermal crystallization process, the multiple melting behaviors were found and each melting endotherm has a different origin. The real-time XRD measurements confirmed that no crystalline transition existed during the isothermal crystallization process. The multiple endotherms were experimentally evidenced due to melting of the recrystallizated materials or the lamellae produced under different crystallization processes. The equilibrium melting point of samples for isothermal crystallization was also evaluated. (c) 2006 Elsevier Ltd. All rights reserved.