Abstract: As the main form of partial discharge in transformer oil, corona discharge will endanger the safe operation of converter transformer. The insulation performance of transformer oil can be improved by nano-modification. However, the research on the modification effect and mechanism of corona discharge under DC voltage is insufficient. Therefore, transformer oil modified by TiO₂ nanoparticles was prepared. Then a series of experiments including corona discharge image shooting, measurement of pulse current and light pulse were carried out with needle-plate defects in nanofluids (NFs). And the charge transport characteristics in NFs were measured. The results show that the breakdown voltage of NFs is increased by 23.8% and the corona discharge is greatly inhibited under a negative DC voltage. Compared with pure oil in 50 kV, the corona luminous area in NFs is reduced by 86.0%. Meanwhile, the frequencies of the optical pulse and current pulse were reduced by 75.6% and 76.3%, respectively, and the amplitudes were reduced by 92.8% and 78.6%, respectively. The reason why nanoparticles inhibit corona discharge is that more shallow traps introduced by nanoparticles accelerate electrons escaping from the ionization region, and its polarization trapping effect weakens the impact ionization of electrons, inhibiting the initiation of electron avalanche. In addition, the formation of negatively charged nanoparticles is equivalent to increase the number and decrease the mobility of negative ions, reducing the electric field strength at the tip of needle. The research in this paper will provide guidance for the suppression method of DC corona discharge and the selection of nanoparticles in transformer oil.