The degradation of polystyrene(PS) in supercritical acetone was carried out in the temperature range of 260 to 300℃ using a batch type autoclave reactor. The total conversion rapidly increased with the increase of temperature from 270 to 290℃. The yield of non-aromatic condensates and the total conversion were affected little by the acetone density, which considerably influenced the product distribution. The yield of styrene was the highest among other products, when the weight ratio of acetone to PS was fixed at 11.2, which was equivalent to acetone density 0.406. The yields of styrene and diphenylbutene (equivalent to dimer of styrene) attained maximum values at a temperature in between 280 and 290 ℃.