Dielectric response measurement is emerging as one of the potential non-destructive tools for transformer insulation diagnosis. Studies have been conducted to derive equivalent circuits for dielectric response of transformer oil/paper insulation system, which are normally based on a linear RC model. However, in practice transformer insulation systems may exhibit nonlinear polarisation behaviour. This paper investigates the nonlinearity of the dielectric response of transformer insulation under high excitation voltages. Polarisation and depolarisation current (PDC) measurement is performed on a distribution transformer under different voltages to study the nonlinearity of its dielectric response. Based on these measurements a nonlinear transformer insulation model is proposed, in which a current controlled voltage source (CCVS) is augmented to the well-known Debye model to approximate the nonlinear effect.