This paper investigates the modelling of dielectric response measurements for power transformer condition assessment. The insulation of a power transformer consists of oil and cellulose (insulation paper). The dielectric properties of both materials generally change during the life of the transformer and dielectric measurements have therefore been applied to assess the quality of the transformer insulation system. Recently, a new method, the Frequency Domain Spectroscopy (FDS) measurements, has been applied which is based on the measurements of the dielectric response of the insulation in the frequency domain. In this paper, the effect of transformer insulation geometry and the insulation system temperature on the transformer insulation properties are studied. Different transformer insulation geometrical arrangements and their temperatures are simulated using the dielectric spectroscopy program MODS from General Electrical company (GE). It is used to model different cases of insulation geometry arrangement by changing the percentage ratio of oil and paper of insulation.