To investigate the difference of dissolved gases under thermal faults between different types of insulating oils, simulated thermal-fault experiments of 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 ℃ were conducted. 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. As the experiment shows, 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℃). From the difference of characteristic gases under thermal faults between vegetable and mineral insulating oils, it suggests that suitable methods of DGA (dissolved gas-in-oil analysis) for specific type of insulating oil should be considered in the fault diagnosis of transformers.