This paper is devoted to a comparative study of partial discharge (PD) characteristics initiated in pressboard/mineral oil and pressboard/vegetable oil insulating systems. We mainly analyze the average number N and maximum apparent charge q ap.max of discharges versus the voltage. The results are discussed with respect to the thickness of pressboard and type of oil. It's shown that for a given voltage and pressboard of 4 mm thickness, any significant difference is observed in the value of N and q ap.max in both mineral and vegetable oils while for a thickness of 2 mm N and q ap.max are much higher in vegetable oil than in mineral oil. As concerns the phase, the maximum of discharge activity moves to the left (i.e. towards the low phases) when increasing the voltage. This would indicate that the influence of the electric field and capacitive effects decrease when the thickness increases. As concerns the phase, the maximum of discharge activity moves to the left (i.e. towards the low phases) when increasing the voltage.