Dissolved gas in oil analysis has been used for fault diagnosis of oil immersed insulation. In this paper to improve the reliability in deciding the degradation causes of the oil immersed insulation, we carried out electrical and thermal degradations for the insulations, and analyzed the characteristics of dissolved gases distribution on each situation. As a result more reliable faults discrimination is possible if we use the interrelation of factors like key gases and gas compositions of hydrocarbon gases and ratios of CO/$CO_2$.