This paper present the degradation characteristics of transformer oil. Degradation accelerates in the presence of moisture and high temperature. So it is important to investigate dominant factors of transformer oil. Sample of four working transformer of 50 to 150 MVA and voltage class 400/220Kv is taken for the test. In this paper investigation is done with the help of graph and shows the characteristics between different testing parameters. So we can easily see the characteristics between different parameter and their ageing effects. These characteristics can be used as trend for monitoring and detect abnormalities. For the incipient fault analysis historical data of DGA test is used to calculate transformer health. In this paper
transformer condition are evaluated by ratio method.