In this research, nano-TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> of various diameters is added to cellulose and pressboard is made to enhance its anti-thermal aging characteristics. After thermally aging of the pressboard samples, partial discharge inception voltage (PDIV), AC breakdown strength and relative permittivity are measured. The results show that the dielectric properties of the pressboard made with 10 nm TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> has the highest AC breakdown due to closer match of the relative permittivity. The mechanical properties are higher than the unfilled cellulose samples. The change observed of the surface microstructure of the samples show that aging resistance of the filled samples are better than the unfilled samples.