Mineral oil is the most widely used to insulation material of transformer because of the long experience and low cost. Although mineral oil has many advantages, it posses fire hazard when it use in the buildings. Thirty years ago, fire resistant hydrocarbon fluids and mold transformer were developed such as PCBs, Silicone oil, HMWH and Cast resin transformer. Cast resin transformer is widely used in the almost buildings in Korea. Recently, the application of ester fluids have been reported. This paper compares to advantages and disadvantages of the cast resin and ester fluid transformers.