In transformers several kinds of cellulosic insulating materials are used, e.g. transformer board. In these materials insulating oil must be easily impregnated. Several reports dealing with oil impregnation in transformer boards have been published. However, few reports indicate how to estimate oil impregnation depth. In order to estimate oil impregnation in transformer boards, changes in oil impregnation depth and the increase in internal pressure for transformer boards were measured, varying temperature and atmospheric pressure. It was found that oil impregnation depth increases with time and that the internal pressure in transformer boards increases due to oil impregnation. However, the measured internal pressure did not coincide with the calculated internal pressure without compensation of water vapor in transformer boards. Moreover, it was found that when temperature is increased, insulating oil is more easily impregnated due to the decrease of viscosity.