Abstract: The transformer oil insulation will deteriorates under the effect of thermal, electrical and chemical factors and under water which is one of the by-product of ageing. In order to evaluate the dielectric properties of sealed and unsealed dissipation factor (tan delta) capacitance measurement at power frequency (50 Hz) has commonly used to diagnose the insulation integrity and condition of transformer. The main drawback of this technique is that the measurement result obtained represent the status of the complete transformer insulation system. Hence, in this paper experimental investigation has been done by studying the dielectric property change (tan delta, dielectric constant and acidity) of different transformer oil sealed and unsealed to calculate affecting parameters namely moisture and ageing of both the sample (sealed and unsealed). A simulation model of the combined transformer insulation has been used to correlate the tan delta, acidity of each insulating material differentiate the sealed and unsealed value of a transformer.