Semiconducting polymer-based composites with nonlinear electrical behavior are usually used as field grading materials in high-voltage applications, such as generators, cable accessories etc. These types of composites are typically based on polymer matrices that are filled with semiconducting powders such as ZnO. The present work describes the preparation of flexible semiconducting nanocomposites by incorporating nano-scaled ZnO particles into a silicone-based resin. The nanocomposites were obtained by a direct mixing approach using a low viscous silicone resin (RTV type) and various concentrations of ZnO nanoparticles. The influence of particle concentration, particle size and dopants (like Al) on the electrical behavior (nonlinear current-voltage characteristics and conductivity) as well as on the physico-chemical and thermo-mechanical properties were investigated. In addition, possible conduction mechanisms, which lead to the nonlinear behavior, are discussed.