This paper deals with the effect of the nano filler loading in epoxy resin (EP) on creepage discharge properties of the epoxy/silica nanocomposite (EP/S-NC) in mineral oil under standard lightning impulse voltage application using needle-bar electrode arrangement. Experimental results allow to estimate discharge occurrence probability (P d ) at each voltage level. The relative permittivity ε’ r . and dielectric loss are measured at 50 Hz–5 MHz for EP/S-NC with a good dispersion of nanoparticles in the base epoxy matrix. As a result, it is found that P d decreases with increasing the filler loading up to 1 wt%, and then increases. Namely, the nano filler content of 1 wt% produces the best insulation performance in terms of Pd. The relative permittivity of the composite decreases with decreasing the filler loading below 1 wt%, while further increasing nano silica filler loading results in increased ε’ r above 3 wt%.