Nanocomposites have gained interest as insulating materials due to their excellent ability to resist electrical discharges. However, the poor adhesion of nanoparticles with polymer may reduce their capability towards discharge resistances. In the present study, atmospheric pressure plasma treatment was used to treat SiO2 nanoparticles to improve the dispersion of silicone rubber (SiR) based nanocomposites. Results show that the plasma treated SiO2/SiR nanocomposites had slower electrical treeing growth compared to silane treated SiO2/SiR nanocomposites. In addition, there were significant improvements in the electrical tree performances of the plasma-treated nanocomposites with the increase in nanofiller concentration. Furthermore, the dispersion was improved with less agglomeration found in the plasma treated nanocomposites.