Structural transformation from a linear chain structure to crosslinked chain structure, occuring during the thermal stabilization stage of carbon fiber manufacture, was followed through the use of infrared spectroscopy, differential scanning calorimetry (DSC), elemental analysis and gel-fraction measurements. The results obtained from the analysis of IR spectroscopy showed the gradual and continuous loss of intensity of the nitrile ($C{equiv}N$) vibration at 2242 $cm^{-1}$ closely associated with the cyclization reactions whereas the intensity loss of the methylene ($CH_2$) groups vibration around 2920-2939 $cm^{-1}$ has been attributed to the loss of hydrogen atoms as part of the dehydrogenation reactions. The dehydrogenation index, evaluated using the absorbance ratio of $A_{1452}/A_{1368}$, also indicated the gradual loss of hydrogens in agreement with decreasing hydrogen content with progressing stabilization process. IR spectroscopy also showed the emergence of new absorption bands attributed to the formation of crosslinked ladder-like structure in the 1590-1600 $cm^{-1}$ region. The amount of newly formed crosslinked structure was characterized using DSC conversion index, IR conversion index and gel-fraction measurements. The results are compared and discussed in detail.