To investigate the difference of dissolved gases under thermal faults between vegetable and mineral insulating oils, simulated thermal-fault experiments were conducted for camellia insulating oil, Envirotemp FR3 fluid, 25# mineral insulating oil and the paper-oil insulation systems at the temperature of 90-250℃ and 300-800℃. The dissolved gases in oils under thermal faults were analyzed using chromatography to obtain the components and percentage contents of the specific gases as well as their relationship with the temperature. The experiment results show that the main dissolved gases under thermal faults of the camellia insulating oil are H₂ and C₂H₆, and that of the FR3 insulating oil is C₂H₆; for the mineral insulating oil, the main dissolved gases are H₂ and CH₄ under thermal faults at lower temperatures (<300℃), while those are CH₄ and C₂H₄ at medium and higher temperatures (≥ 300℃). The identification of different types of specific gases under thermal faults between vegetable and mineral insulating oils suggests that suitable methods of DGA (dissolved gas-in-oil analysis) for specific type of insulating oil should be developed in the fault diagnosis of transformers.