Aging of insulating material in a pole transformer has been studied by performing accelerated thermal aging test as well as arc discharge test. Dissolved gases were extracted by air bubbling method. Concentrations of dissolved gases were modified by extraction ratios of each gases in insulating oil. Aging of insulating material was proceeded by both thermal degradation and oxidation reaction. Both of the reactions in insulating oil were shown to follow zeroth order kinetics. The activation energy for the thermal degradation was estimated to be about 25,800 cal/mol and 20,600 cal/mol for oxidation reaction. Gas patterns considering H₂, CH₄, C₂H₆, C₂H₄ and C₂H₂ for accelerated thermal aging test showed H₂ domination pattern. Species of maximum hydrocarbon concentration shifted from CH₄ to C₂H₆ and C₂H₄ as the reaction temperature increased from 200℃ to 250℃. During the initial stage of discharge test H₂ was the major component. However, C₂H₂ which was hardly detected in thermal aging test became the maximum concentration species as the discharge time increased.