Dielectric polymers which can operate at high temperature (> 150 °C) and high electric fields with high performance such as high charge/discharge efficiency and dielectric strength are required for many capacitor applications including energy storage and electric insulators. These dielectric performances are controlled directly by charge injection and conduction at high fields and high temperature. Nanoparticles, through interfacial effects, can markedly influence dielectric properties of the matrix. Here we report a research on a significantly enhanced charge/discharge efficiency in high glass transition temperature polymer, i. e. polymers in PEEK family by doping with very small amount of nanoparticles. The presence of nanoparticles reduces the leakage current of the polymer matrix in the conditions of high electric field and high temperature. Our results demonstrate a practicable way to enhance the performance of polymer-based capacitors working at high temperature and high electric field.