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In this paper, carbon black-filled high dielectric constant composites were evaluated as the candidate materials for embedded capacitors. Carbon black was selected as the filler due to its large surface area and its wide range of electrical properties based on its surface chemistry, particle size and aggregate structure. Six different types of carbon blacks were evaluated. With a proper filler loading level and good dispersion, high dielectric constants over 1000 were observed for four out of the six types of carbon. The dispersion of carbon black is critical to obtain high dielectric constant carbon black composites. When the carbon black is well dispersed, the large surface area of tiny carbon black aggregates serve as the electrode surfaces of numerous small capacitors. The capacitor network within the composite can thus give a large capacitance and dielectric constant. For a highly conductive carbon black CBD3, a high dielectric constant over 13,300 (@10 kHz) was achieved, and for a relatively low conductivity carbon black CBC2, a dielectric constant of about 2,300 (@10 kHz) was obtained. A higher conductivity carbon black usually gives a higher dielectric constant; however, its composites are more difficult to process since the composites have a narrower composition window near the percolation threshold due to its high structure and large surface area. The filler loading level required to reach high dielectric constant is much lower than that of ceramic composites, which enable carbon black composites to have good mechanical properties. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were used to characterize the structure of carbon and their composites, respectively, in order to correlate the structure of carbon blacks and the morphology of their composites with the corresponding material dielectric properties.