Given that the carbon fiber production is an energy intensive process, new approaches are investigated. One option might be dielectric heating, but in order to design an efficient applicator, the knowledge of the temperature dependent dielectric properties is a requirement. In this paper the focus is on the dielectric measurements of the Polyacrylonitrile (PAN) precursor fibers and stabilized fibers. A measurement system was developed that is capable to monitor in-situ changes of the dielectric properties during the stabilization process of the PAN fiber. First results show that the losses of the PAN fiber are strongly varying with the temperature. Additionally, the change in the dielectric properties due to the chemical transformation during the stabilization process can be tracked. The strong temperature dependency is decreasing with a higher degree of stabilization.