Abstract <p>This paper investigates the transformer oil-based Fe₃O₄-coated SiO₂(Fe₃O₄@SiO₂) core/shell nanofluid under both positive and negative impulse breakdown voltage test. The nanoparticles investigated were prepared with two different shell thicknesses, and one sample was introduced with surface modification. Nanofluid samples were prepared at a concentration of 0.04 g/L. A two-step method was suggested for the preparation of the nanofluid samples: nanoparticles were calibrated with a sensitive balance, then mixed with the base oil and dispersed using ultrasonicate probe. Fourier Transform Infrared Spectroscopy (FTIR) was used to characterize the nanoparticles. The spectrums for unmodified and modified nanoparticles detected Fe–O bond, Si–O–Si bond, and oleic acid existence. Under positive impulse voltage, the sample (BF) improved compared to the base oil. Under negative impulse voltage, the sample (CF) showed the highest improvement percentage compared to the base oil. The physical discussion explored potential reasons for these improvements under impulse voltage. The nanoparticle electron capturing effect was identified as the reason for improving nanofluid samples.</p>