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Simultaneous differential scanning calorimetry (DSC) and luminescence (LUM) measurements of polyacrylonitrile revealed that a precursory reaction progresses below the well-known endothermic cyclization reaction (i.e., peak temperature ca. 280 °C). The intensity of LUM increased above 90 °C, which is around the glass transition temperature. After the peak at 180–190 °C, the LUM intensity decreased with temperature up to 250 °C, i.e., below the onset temperature of cyclization reaction. Thermal signals cannot be seen on the DSC curve in the temperature range of the LUM peak. The activation energy (Ea) estimated from the heating rate dependence of the LUM peak temperatures was 122 kJ mol−1. However, the Ea obtained from the well-known exothermic peak temperatures of DSC measurements was 190 kJ mol−1. Electron spin resonance and nuclear magnetic resonance also support the chemical structure of the luminescence species. FT-IR and X-ray diffraction were also performed to confirm the change in chemical and physical structural. Finally, ab initio calculations suggest that thermally stable heterocyclic radical species with two to four fused six-membered rings are the main products of the precursory reaction.