Ester dielectric liquids were found to overcome mineral oils in terms of flash point and environmental aspects. We study Ester oils behavior through partial discharge inception voltage (PDIV) and compare it with mineral oil. We conduct experiments on two types of oil, ester oil (fatty acid ester made from vegetable oil) and mineral oil so that the comparison can be made. We use the Needle-To-Plane electrodes configuration to get the PDIV and breakdown voltage (BDV). The device we made to test PD is a vertical acrylic tube with a high voltage electrode at top and a ground electrode at bottom and filled with the tested oil, we change the gap length for different readings for both types of oil. The natural ester oil is showing a good potential for medium voltage transformer insulation with slightly higher PDIV than mineral oil.