Mineral oil (MO) works as an important electrical insulation and coolant in transformer which is non-biodegradable and nearly running out. Therefore, for sustainable and environmental concern, an alternative biodegradable insulating oil that potential to replace the mineral oil is introduced. In view of that, the breakdown strength characteristic of Refined Bleached Deodorized Palm Oil (RBDPO) and MO mixtures were investigated by varying the mixing percentage of RBDPO from 0% to 100% at 40oC. The results showed that the breakdown strength of the oil mixture abruptly decline to the minimum breakdown voltage of 50 kV at 20% of RBDPO mixture and gradually increased when the ratio of the RBDPO is added. The highest breakdown strength is achieved 87kv at 80% of RBDPO content. The result of kinematic viscosity is also presented.