Thermal degradation of polyacrylonitrile (PAN) has been studied by combined differential thermal analysis, thermogravimetry, and XRD analysis. On the basis of the experimental observations, it has been suggested that the intramolecular polymerization of nitrile groups to a conjugated polyimine structure is initially limited to only the amorphous zone of PAN, which involves little evolution of off-gases. Polymerization of nitrile group in the crystalline zone starts with the melting of crystallites with the evolution of off-gases from the amorphous-crystalline zone interface.