Degradation of polystyrene was carried out in supercritical n-hexane under reaction temperature ranging from 330 ℃ to 390 ℃, pressure ranging from 30 bar to 70 bar and reaction duration of 90 min. The conversion of polystyrene increased with rising temperature and pressure. The degradation performance was influenced by the temperature rather than applied pressure. Polystyrene rapidly degraded in 30 min after reaching a prescribed temperature ranging from 350 ℃ to 390 ℃. At a prescribed temperature of 390 ℃, the degree of degradation was higher than 90%. The degradation reaction was examined experimentally at a relatively low temperature of 330 ℃. The degradation of polystyrene by using supercritical n-hexane has been found to be more effective compared to general pyrolysis (thermal degradation). Among the selectivity of liquid products, that of a single aromatic ring group like styrene at 390 ℃ increased up to 65% in 90 min. It was found from the analysis by a gel permeation chromatograph (GPC), that high molecular-weight compounds decreased but oligomers; increased with rising temperature.