Abstract: Under the condition of tensile stress, dielectric strengths of resin and unique directional glass fiber reinforced resin composites were tested, and a relationship between dielectric strength and tensile stress of the resin and glass fiber/resin composites was studied. A negative exponential equation for expressing the relationship between dielectric strength and tensile stress for the resin was proposed and then was experimental proved. It has been found that the interface of the fiber and the resin plays a dominant role on the dielectric strength of composites.