Dielectric materials with high electrical energy density, low loss, and high thermal stability are desirable for a broad range of modern power electronic systems. Here, we investigate the conduction mechanism at high temperatures and high fields in a semi-crystalline poly(tetrafluoroethylene-hexafluoropropylene-vinylidene fluoride) (THV) terpolymer, which has been shown to be attractive for high temperature and high energy density capacitors. In order to suppress conduction at high temperature and high electric field, alumina (Al2O3) nanofillers were added to the THV polymer matrix. Experimental results show that the Al2O3 nanofillers are very effective in reducing the conduction current, and at 125°C nanocomposites exhibit more than two orders of magnitude reduction in conduction compared to the neat polymer. Continuum and particle simulations were carried out to understand the leakage conductivity, and simulation results agree very well with the measured data. Hopping conduction has been identified as the dominant conduction mechanism.