In this investigation, four vegetable oils namely: linseed, rapeseed, radish and used frying oil were subjected to chemical transesterification to be used as dielectric fluids. thus, transesterification of these natural esters using pentaerythritol led to the formation of the corresponding polyesters. Each polyester produced was divided into 2 parts: the first part was tested as dielectric fluid according to (American Society for Testing and Materials ASTM) methods and the results were compared with International Standard Specifications (International Electrotechnical Commission IEC). The second part was treated with DBPC (2,6-di-tert-butyl-p-cresol) as an antioxidant additive and then tested as dielectric fluid according to ASTM methods and the results were compared with IEC specifications again. Linseed polyester and used frying oil polyester gave better results.