The surface of oil immersed pressboard (PB) influences partial discharge (PD) inception voltage and creepage discharge properties. This paper deals with surface modification of insulating paper by thin solid layer coating such as epoxy resin, Teflon and nano size silica/epoxy resin composites on PB surface to improve electrical insulation properties of oil/pressboard composite insulation system. Five kinds of specimens are prepared with different coating materials to investigate creepage discharge initiation and propagation under positive standard lightning impulse voltage (1.2/50 µs) using a rod-plane electrode arrangement in mineral oil. Creepage discharge properties of conventional PB in oil/PB composite insulation system are compared with that of the solid layer coated PB in terms of discharge final propagation length (l_d) and PD inception voltage (PDIV) as an inception probability (P_i). The results show that PDIV of the solid layer coated PB increases at all applied impulse voltage levels compared with that of conventional PB. Especially, when the filler content of nano silica added in base epoxy was SP-0.2wt%, PDIV characteristic was improved. The effect of solid layer coating of PB surface on the creepage discharge properties is also discussed in terms of the effects of introduced deep electron traps due to the nano composites coating as well as the drying effect to remove water content and the surface roughness of PB.