The thermal decomposition mechanism of ester insulating oil was investigated using quantum chemistry computing. The bond dissociation energy of each ester's molecular bond was calculated, activate energy for generation radicals estimated. Gibbs free energy difference before and after bond dissociation was calculated at 300, 400, 500, 600, and 700°C. The reaction rate at each temperature was evaluated, and the ratio of each radical was estimated. Furthermore, the reaction pathway in which radicals generate gases (H2, CH4, C2H6, C2H4, C2H2) used in DGA was searched by transition state (TS) search and intrinsic reaction coordinate (IRC) analyzing. Finally, the Gibbs free energy for these reaction pathways was calculated and the gas generation rate at each temperature was estimated. Calculated gas generation characteristic trends are consistent with the data of local heating experiments previously reported.