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In this paper we investigated a charge accumulation of a pressboard (PB) caused by streaming electrification in transformer oil. In positive charged oil generated by filter circulation, PB in the oil had a positive charge. We evaluated the PB charge quantity by measuring a voltage or floating electrode in the PB which was inserted between two parallel plane electrodes. The results are as follows. When the moisture content in PB increased, the PB charge quantity decreased. Charge density ratio between the PB and oil was varied by the charge density in oil. So under the same moisture content in PB the higher the charge density in oil was, the smaller the ratio was. Also we investigated AC field effect on the PB charge accumulation. It was found that the AC electrical field did not have an effect on the accumulation for existing AC field below 1.0 kV/mm in the oil.