The current research paper deals with contribution to the worldwide problem of transformers which are essential parts to maintain the power flow in the electrical power system, the stability is significant for the reliability of the whole supply. The oil used in all transformers is used for insulating and cooling purposes. Degradation of transformer oil occurs because of the ageing, high temperature and chemical reactions such as the oxidation. It is also affected by contaminants from the solid materials. Therefore, the oil condition must be checked regularly and reclaimed or replaced periodically, to avoid the sudden. In this work is devoted to study the transformer oil behavior under AC voltage at industry frequency (50 Hz). The mineral used mineral oil Borak 22 is examined for different parameters such as, the distance between electrodes and geometry of electrodes. The experiment results concerning the evolution of the breakdown voltage into new oil and another old. The results showed that the spacing of the inter-electrode distance causes an increase in the breakdown voltage of the oil, and that the point-plat electrode configuration was the worst form of the configurations. Finally, the used oil was better than new oil. He current research paper deals with contribution to the worldwide problem of transformers. \textcopyright Springer Nature Singapore Pte Ltd. 2021.