Thermal decomposition of the copolymer of ${alpha}$-Methylstyrene(AMS) with Acrylonitrile(AN) was investigated. The copolymer was synthesized in a continuous stirred tank reactor(CSTR) at $80^{circ}C$ using toluene and benzoyl peroxide(BPO) as solvent and initiator, respectively. The reactor volume was 0.3 liters and residence time was 3 hours. The activation energy of thermal decomposition was in the ranges of $34{sim}54$ kcal/mol for AMS with AN copolymer. The thermogravimetric trace curves were well agreed with the theoretical calculation.