Thermal decomposition kinetics of diglycidyl ether of bisphenol A (DGEBA)/4,4'-methylene dianiline (MDA) system with rubber-modified MDA was studied by the methods of Ozawa, Kissinger, and Friedman, and the kinetic parameters were compared. The thermal decomposition data of the cured epoxy resin were analyzed by thermogravimetric analysis (TGA) at different heating rates. TG curves showed that the thermal decomposition of the epoxy system occurred in one stage regardless of rubber-modified MDA content. The apparent activation energies for the DGEBA/MDA system with 10 phr of rubber-modified MDA, as determined by the Ozawa, Kissinger, and Friedman methods, are 184, 182, and 222 kJ/mol, respectively. The thermal stability of the epoxy system increased with the increasing content of rubber-modified MDA, which has four benzene rings with high thermal resistance due to the resonance structure.