In this study, a mineral oil substitute which employs waste soybean oil that has been chemically altered through transesterification and epoxidation reactions to achieve the desired properties that meet ASTM standard requirements for power transformers is proposed. After going through the transesterification and epoxidation processes, the performance of waste soybean vegetable oil for use in electrical transformers has been assessed. For fresh mineral oil FMO, used mineral oil UMO, and recycled mineral oil RMO which is produced by mixing with synthesized soybean vegetable oil SSVO with different concentrations , the physical properties (dynamic viscosity and flash point), chemical properties (water content and acidity), and electrical properties (break down voltage) were measured . According to these measured data, the physical characteristics of the RMO at the mixing proportion (7:3- UTO: SSVO) are higher than those of the FMO and UMO, respectively, while the water content of RMO is significantly higher than that of FMO but lower than that of UMO. Additionally, the breakdown voltage values of RMO show an increase over UMO but it is lower than FMO and the acidity of RMO is lower than UMO and higher than FMO, and all previous measurements meet the limited values of ASTM . The current study suggested synthesized soybean vegetable oil as a sustainable method of utilizing waste resources, reducing environmental pollution, and promoting environmental and economic benefits in addition to being a replacement cooling medium for liquid-filled transformers.