Abstract: Ester insulating oils are increasingly being used in power transformers as environmental-friendly insulating fluid. However, there are significant differences in the physicochemical and electrical properties of different ester insulating oils, and their gas generation laws under the same fault type are also somewhat different. In this paper, natural, modified and synthetic esters are used to study the gas generation law and differences at different discharge energies, and the correlation between characteristic gas content and discharge energy is analyzed. The results show that there are some differences in the gas generation law of ester insulating oil with different molecular structures and compositions. Natural esters are prone to produce more H₂ and C₂H₆, while modified esters and synthetic esters are prone to produce more C₂H₄ and C₂H₂. Differences in the molecular structure of the insulating oil and the dissolution characteristics of the gases are the main reasons for the dissolved gases in oil. With the increase of discharge energy, the relative percentage contents of H₂, CH₄ and C₂H₆ decrease, while the relative percentage contents of C₂H₂, C₂H₄ and CO increase. In addition, the severity of the discharge fault can be qualitatively determined by fault gas volumes per unit fault energy, the ratio of C₂H₂/H₂ and the CO content, which can provide reference for the state diagnosis of ester insulating oil transformers.