Abstract: Polytetrafluoroethylene(PTFE), polyethylene(PE), polymethyl methacrylate(PMMA) and other similar medium materials are widely used for making of output window of high power microwave(HPM), due to their favorable wave-transparent and mechanical properties. But there are few studies conducted on the influence of different processing technologies on dielectric properties of these materials. In this article, the basic dielectric parameters of different medium materials processed with different technologies are measured with high-pressure bridges and high resistance meters, and the surface electrical trap densities of these materials at the low energy level of 0.8-0.9 eV are measured with the adoption of electrostatic induction method. Medium breakdown experiments with waveguides under S band, 800 MW and 100 ns microwave pulse are conducted, and the effects of different material surface finishes on window surface breakdown are studied. The results show that, the baking technology to some extent reduces materials resistance to breakdown, and causes more dielectric losses; the scratches parallel to the direction of electric field form the channel for material surface breakdown, leading to more serious breakdown.