Thermal stabilization of two kinds of polyacrylonitrile (PAN) fibers was studied by the combination of differential thermal analysis (DTA), thermogravimetric (TG), wide angle X-ray diffraction (WAXD), tension testing, elemental analysis and microscopic examination. The cyclization reaction in fiber P1 occurs at higher temperature with a concentrated heat release, while its incorporation of oxygen is earlier and more rapid than that in fiber P2. Skin-core structure was observed in the as-stabilized fiber of P1, but not in that of P2. During heat treatment, the stress in fiber P1 is about 20 MPa higher than that in fiber P2. The results indicate that crystallinity and content of acidic comonomers in PAN fibers are the essential property indices which influence thermal stabilization significantly. Early initiation and sufficient cyclization are important preconditions for obtaining stabilized fibers with uniform microstructure.