Thermal decomposition of the copolymer of styrene(St.) with n-butylmethacylate(BMA) were investigated. The copolymer were obtained in a continuous tank reactor(CSTR) using toluene and benzoylperoxide, as solvent and initiator, respectively, at 80 ℃. The volume was 0.3 liters and residence time 3 hours. The thermal decomposition followed the second order kinetics for St./BMA copolymers. The activation energies of thermal decomposition were in the range of 24~47 kcal/mol for St. with BMA copolymer and a good additivity rule was observed against composition copolymer. The thermal decomposition followed the second order kinetics for St./BMA copolymers. The thermogravimetric trace curve agreed well with the theoretical calculation.