Oil formation rate, composition of crude oil and formation of side products such as α-methyl styrene, ethyl benzene, benzene, toluene, dimer and trimer on thermal degradation of polystyrene were affected by various factors. Especially, formation of organic residue formed during reaction gave an important influence on formation of oil and composition of crude oil. Also, composition of formed crude oil showed a significant difference on reaction time. These results were caused by organic residue and carbonized solid formed during continuous reaction. Increase of residue and carbonized solid gave a decrease of yield of styrene and an increase of formation of α-methyl styrene, ethyl benzene, benzene, toluene. New reaction system was proposed for continuous operation at the thermal degradation of polystyrene.