The addition of fly ash as a silicone rubber filler can increase the dielectric strength of the material, besides that the electrical test values are obtained from variations in the composition of the amount of fly ash in Room Temperature Vulcanization (RTV) silicone rubber. In this research, further studies were carried out to determine the optimization value of the composite characteristics of various variations in the composition of the amount of fly ash in RTV silicone rubber using the quadratic regression method and using RMSE to evaluate the level of accuracy of the model from the optimization results. This study carried out three types: breakdown voltage testing, volume resistivity testing, and surface resistivity to the amount of fly ash in RTV silicone rubber composite. After the test values were obtained using American Standard Testing and Materials (ASTM), composition optimization was continued using the quadratic regression method on the optimum composition of fly ash in RTV silicone rubber. The results obtained for the best composition of the three types of testing, the results of the breakdown voltage test obtained an optimum composition value of 50.88% with an RMSE value of 1.15 with an average percentage of 8.10%, then the volume resistivity test results obtained an optimum value composition of 29.90% with an RMSE value of 1.91 with an average percentage of 10.56%. The surface resistivity test results obtained an optimum composition value of 0.49% with an RMSE value of 12.86 with a percentage average of 6.52%.