Synthetic ester nano silica fluid was synthesized and characterized through various physico-chemical diagnostic techniques including the measurement of dielectric constant and ionic mobility in order to demonstrate its enhanced dielectric properties and use it as insulants in transformers. The use of UHF technique for identification of incipient discharge activity in synthetic ester nano fluid under AC, DC and harmonic AC voltages is discussed. Statistical analysis of the breakdown characteristics obtained with ester nano fluids would be useful for insulation designers. Mixed oil formed with ester oil and mineral oil were characterized for its stability, miscibility and dielectric performance, to assess its suitability as insulants in transformers.