The test vessel, electrode system and test procedure for OIP were set up according to IEC 60093 and IEC 60250. We concluded that, first, relative permitivitty and dielectric dissipation factor of oil immersed pressboard are highly influenced by temperature. The change of dielectric dissipation factor is significant only for temperature ı 90eC. Second, the relative permitivitty and dielectric dissipation factor of pressboard do not depend on the electric field stress for range 0.125 kV/mm-5 kV/mm