Abstract: This paper investigates the effect of two organic additives, including dimethylaniline (DMA) and trichloroethylene (TCE), and three inorganic nanoparticle additives, including zinc oxide, silicon dioxide, and alumina, on the dielectric strength of transformer oil. The power frequency breakdown voltage and lightning impulse voltage of transformer oil are measured. The results show that both types of additives effectively improve the dielectric strength of transformer oil under appropriate mixing ratios, among which zinc oxide nanoparticles show the best overall modification effect.