In this study Investigation of intrinsic breakdown of transformer oil insulation has been done. Three samples of transformer oil insulation were tested under impulse voltage stress as well as under ac voltage. These three samples were used, non-treatment, and treatment transformer oil from a same company. The results show that under impulse voltage of 1.2/50 μs oil under tested remarkable difference between used oil and new oil of about 45kV and 87 kV, respectively. This difference results may show that the rise time of applied voltage was not representatively fast to investigate the intrinsic dielectric strength of sample under tested. However, new treated oil as well as non-treated has the same order of breakdown voltage under impulse voltage stress. Moreover under power frequency applied voltage, three sample under tested have notable difference breakdown voltage which the treated oil insulation has the highest breakdown voltage of about 50 kV. These are reasonable result and can be explained by using all of at this juncture developed breakdown mechanism theories.