Abstract: The present paper focuses on the metabolism of antioxidant T501 in the transformer oil caused by Mycoplana and the relationship among the quantity of Mycoplana, the majo r components of the oil (such as alkane, cyclanes and aromatic hydrocarbon) and dielectric loss. The experiment result indicates that Mycoplana could grow taking antioxidant T501（2，6-ditertiary-butyl-pcresol） as the sole carbon and energy sources. With the quantity of Mycoplana put into the oil inc reasing gradually, the content of T501 became more and more lower, at the same time, the contents of alkane, cyclanes and aromatic hydrocarbon in the transformer oil also became more and more lower, which led to the increasing of dielectric loss.

Key words: transformer oil, Mycoplana, antioxidant, dielectric loss