Dielectric properties of synthetic ester oil are investigated in wide frequency range (10 mHz-1 MHz) and temperature (-60°C -200°C) ranges. The aim is to determine the ability of liquid insulation, to be used as encapsulating materials for high temperature power electronics modules. As far as the ester remains in liquid state (above approximately -16°C), a typical frequency spectrum of the dissipation factor tan δ with two parts is observed. At low frequency, dielectric losses (tan δ > 0.10) are due to ionic conduction, and at high frequency losses are mainly due to dipolar relaxation. When temperature is increased, losses increase at low frequency, and decrease at high frequency. Results show that the liquid conductivity increases by more than 3 orders of magnitude between room temperature and 200°C. A slight decrease of permittivity is also observed, correlated to the lowering of density at high temperature. This behavior totally changes when the ester becomes a gel at low temperature. Dielectric relaxations reminiscent of solid materials then appear.