The thermal degradation of poly(methyl methacrylate) (PMMA) and poly(acrylonitrile butadiene styrene)(ABS) terpolymer as well as their mixtures were carried out using the thermogravimetry and differential scanning calorimetry(DSC) in the stream of nitrogen and air with 50 ㎖/min at the various heating rate from 4 to 20℃/min and temperature from 200 to 300℃ The values of activation energies of thermal degradation determined by TG and DSC in the various PMMA/ABS mixtures were 34∼58Kcal/㏖, 35∼54 Kcal/㏖ in the stream of nitrogen. The values of activation energy of ABS20% mixture was appeared high in camparison with addition rule. According to increasing the composition of ABS, the temperatures of glass transition and initial decomposition temperature were increased. PMMA/ABS mixtures by the analysis of in frayed spectrophotometer were decomposed by main chain scission in the stream of nitrogen.