Solid-state (ss) NMR spectroscopy was applied to study the stabilization process of 30 wt % C-13-labeled atactic-polyacrylonitrile (a-PAN) heat-treated at various temperatures (T-s) under nitrogen and air. Direct polarization magic-angle spinning (DP/MAS) C-13 NMR spectra provided quantitative information about the functional groups of stabilized a-PAN. Two dimensional (2D) refocused C-13-C-13 INADEQUATE and H-1-C-13 HETCOR NMR spectra gave through-bond and through-space correlations, respectively, of the complex intermediates and final structures of a-PAN stabilized at different T-s values. By comparing ID and 2D NMR spectra, it was revealed that the stabilization process of a-PAN under nitrogen is initiated via cyclization, while the stabilization under air proceeds via dehydrogenation. Different initial processes lead to the isolated aromatic ring and ladder formation of the aromatic rings under nitrogen and air, respectively. Side reactions and intermediate structures are also discussed in detail. Through this work, the stabilization index (SI) was defined on the basis of the quantified C-1 and C-3 DP/MAS spectra. The former reached 0.87 at T-s = 370 degrees C, and further higher T-s values did not affect SI; however, the latter continuously increased up to 0.66 at T-s = 450 degrees C. All of the experimental results indicated that oxygen plays a vital role on the whole reaction process as well as the final products of stabilized a-PAN.