Abstract: The composites based on poly(methyl methacrylate) (PMMA) and multiwalled carbon nanotubes (MWCNT) were prepared by a solutionblending method, and their thermal degradation processes under different atmosphere were investigated by a thermogravimetric method. The kinetic parameters of thermal degradation were obtained by the Flynn-Wall-Ozawa’s, Kissinger’s and Friedman’s methods. The results indicated that the initial degradation temperature of composite containing 3 wt % increased by 54.62 ℃ in nitrogen and 70.4 ℃ in oxygen in comparison with pure PMMA, and its maximum heat loss rate was also improved to some extent. It was concluded that the introduction of MWCNT could effectively improve the lowtemperature thermal stability of PMMA, especially in an aerobic environment. However, the improvement in hightemperature thermal stability was not so significant. The values of activation energy (Ea) calculated by Kissinger's, flyn-wall-ozawa's and Friedman's methods were in good agreement with each other. The composites achieved a maximum improvement in Ea when 3 wt % of MWCNT was added into PMMA. In addition, the order of reaction (n) deduced by the Friedmans method for the composites was approximately 1.5 in nitrogen and 0.9 in oxygen.