In this study, the effect of heat-treatment on the change of fine structure of high strength PAN fibers with different molecular weight ranging from 1. V. = 2.5 dl/g to 5.5 dl/g was investigated. The activation energies for the thermal reaction increased with increasing molecular weight. The PAN precursor with low molecular weight exhibits a higher aromatization(A. 1. value) due to its rapid cyclization during heat treatment. The length change of FAN fibers during heat treatment appears to show that the chemical reaction occurs between intramolecular or intermolecular chains. The maximum apparent crystalline size of PAN fibers tends to increase with increasing molecular weight and with heat treatment temperature. Consequently, the PAN fibers with a higher molecular weight exhibit a transition from AN unit to ladder structure at a higher temperature and cause slow cyclization during heat treatment.