Abstract: The breakdown characteristics of transformer oils in DC electric field are one of the key factors to decide the safe operation of equipment. In this paper, the breakdown voltages test methods of transformer oils at home and abroad are briefly introduced. In HVDC electric field, the relevance of electric field uniformity and breakdown voltage, and the performance of electric breakdown of transformer oils with different hydrocarbon compositions as well as the relations between types of aromatics and their content and breakdown characteristics in uneven DC field are investigated. The results show that in uneven electric field, breakdown voltage of transformer oils has a great correlation with their hydrocarbon composition. The higher the aromatic hydrocarbon content in transformer oil, the lower the breakdown voltages in positive and negative polarity. Under the same electrode gap, the negative polarity breakdown voltage is higher than the positive polarity breakdown voltage of transformer oils. The breakdown voltage of transformer oils increases in direct ratio with the increase of electrode gap. The content of polycyclic aromatic hydrocarbons is the key factor affecting DC breakdown voltage of transformer oil; while monocyclic aromatics do not negatively affect the DC breakdown voltage.