Abstract: Gassing tendency of transformer oil with different hydrocarbon compositions in AC, DC electrical field or AC/DC mixed electrical field are investigated according to the standard at home and abroad. The influence of additives on gassing tendency of transformer oils was observed at the same time. The results show that the aromatics content is the key factor, which decide gassing or absorbing gas of transformer oil. At the same temperature and electric field strength, the tendency of gassing or absorbing gas of oils in AC electric field is the largest, the AC and DC mixed electric field takes the second，and the smallest value of gassing or absorbing gas is in the DC electric field. The use of additives has no obvious effect on gassing tendency of oils in specified dosage range.