Insulating oil plays a vital role in transformers as a coolant and electrical insulator. With limited petroleum resources and increasing environmental awareness, environmentally friendly and renewable alternatives are needed. This study aims to analyze the effect of adding olive oil as an additive on the insulating performance of
castor oil (Jatropha curcas) to replace mineral-based transformer insulating oil. Castor oil was chosen because of its biodegradability and abundant availability, while olive oil contains phenolic compounds and antioxidants that can improve chemical stability and electrical properties. Tests were conducted on parameters such as breakdown voltage, viscosity, density, and water content, with varying mixture concentrations referring to SPLN 49-1:1982. The results showed that a composition of 70% castor oil: 30% olive oil provided the best performance. Breakdown voltages of 26.42 kV; 32.20 kV; 34.83 kV; and 34.85 kV produced average values that met the minimum limit of SPLN 49-1:1982 (≥30 kV/2.5 mm). Viscosity decreased significantly to 14.5 cSt, far below the standard maximum limit (≤40 cSt). However, the density of 0.93 g/cm³ exceeded the SPLN 49- 1:1982 limit for new insulating oil (≤0.895 g/cm³), and the water content of 0.025% (250 mg/kg) also exceeded the SPLN 49-1:1982 limit for voltages <170 kV (≤200 mg/kg or ≤0.02%). This indicates that although the breakdown voltage and viscosity meet the standards, the density and water content remain limiting factors, so this mixture does not fully meet the SPLN 49-1:1982 standard for transformer insulating oil.