Kinetic tests on pyrolysis of polystyrene were carried out by thermo gravimetric technique heating the sample at the rates of 0.5, 1.0, 2.0 ℃/min in a stirred batch reactor. The activation energy and the reaction order were determined at conversions from 1 to 100%. The activation energies increased slowly until the conversion increased to a certain extent. Polystyrene was thermally cracked in a semi-batch reactor at 380-400 ℃ for 60 minutes. As the reaction temperature increased, the yields of product oil increased but those of light hydrocarbon oil were almost constant. Also, the selectivity of hydrocarbons corresponding to the styrene monomer and dimer was very high.