Oil-paper insulation has been used widely in many power apparatuses including converter transformers for its excellent insulation performance. As a result of long-term operation, oil-paper degrading and aging gradually, which are the main factors that affect the transformers’ lifespan. In this paper, nano-TiO₂ was added in cellulose to improve its anti-thermal aging ability. The insulating pressboards modified by 10 nm nano-TiO₂ with 5 wt% were selected as samples of thermal aging test with the highest breakdown strength. Accelerated thermal aging experiment was performed on the samples at 130℃ for aged 5, 10, 15 and 20 days, respectively. The breakdown experiments were carried out on the samples under the AC and DC voltage. It is found that the electrical characteristics of the insulating pressboard modified by 5 wt% nano-TiO₂ had increased greatly under different thermal aging stages. The increasing rate of breakdown strength under DC is greater than that under AC. In addition, relative permittivity ɛ and electric resistivity ρ of samples are obtained. The ɛ of modified pressboard is lower than that of the unmodified one, but the ρ is the opposite. It is demonstrated that the electrical characteristics of modified pressboard insulation are improved.