Thermal characteristics and kinetic parameters of polystyrene waste and polystyrene were determined by means of thermogravimetry (TGA) at non-isothermal heating conditions(10 K/min, 20 K/min, 30 K/min, 40 K/min and 50 K/min) both for pyrolysis and combustion processes. Activation energies and reaction orders of the samples were determined by different methods such as Kissinger, Freeman-Carroll, Chatterjee-Conrad, Friedman and Coats-Redfern methods. These methods were compared in terms of their accuracy and the ease of interpretation of the kinetics of thermal decomposition. It was found that the apparent activation energies of the thermal processes were different with variation of analytical methods and experimental conditions. The apparent activation energy of combustion at the atmosphere of 80 ~ 130 kJ/mol was lower than that of pyrolysis at the atmosphere of 90 ~ 150 kJ/mol.