Dissolved Gas Analysis（DGA） system to analyse the gas contents dissolved in transformer oil has ever played an important role in detecting various defects of transformer. However

many demerits exist in the off-line monitoring methods. Aiming at the demerits of the off-line monitoring methods with too complicated operation

an on-line monitoring system based on the gas sensor is presented in this paper. The reason and method which is used to detect the gas dissolved in transformer oil on line are described. The design of gas transducer and the hardwareas well as the software of communication is also introduced.