The activation energy of thermal decomposition in diglycidyl ether of bisphenol A(DGEBA)/4,4'-methylene dianiline(MDA) system was studied by using a dynamic thermogravimetric technique. The decomposition was found in one stage and the activation energy of thermal decomposition was calculated by Ozawa equation. In DGEBA/MDA system, activation energy was about 180 kJ/mol from 0.05 to 0.25 of fractional weight loss. However, at the weight loss of 0.30, it was about 154 kJ/mol and deceased rapidly.