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Polymer based nanocomposites have been of great interest as embedded capacitor dielectrics because of their low cost and excellent compatibility with the organic substrate manufacturing processes. One of the polymer nanocomposites that have been studied is the carbon black/polymer composite, which could give a high dielectric constant over 1,000. In this paper, the dielectric behavior of carbon black CBCA6/epoxy composite was systematically investigated and analyzed. It was found that this composite consistently showed a high dielectric constant of over 1,000 at the proper filler loading levels, however, the dissipation factor of this carbon black composite was high (tanδ>l). Moreover, frequency dependence of the dielectric constant and dielectric loss were observed in the middle frequency range up to 10 MHz. Carbon black is a nano structured material which has a large surface area and a wide range of electrical properties based on its surface chemistry, particle size and aggregate structure, and the polymer-carbon black interfaces are believed to contribute significantly to the high loss and high frequency dependence. Therefore, fundamental studies of the interface and its modification with additives were conducted in this research in order to obtain low-loss ultrahigh-k carbon black composites. A high breakdown field inorganic filler, i.e. fumed silica, was included to the carbon black-polymer formulation in order to modify the carbon black-polymer interfaces and reduce the high conduction dielectric losses of carbon black composites. It was found that the dielectric loss of carbon black composites decreased with the fumed silica inclusion. The fumed silica was also found to significantly change the frequency dependence behavior of carbon black composites.