High dielectric constant (high-k) materials are needed for embedded passives in advanced packaging applications. This work presents a systematic theoretical investigation of the effective dielectric constant, dielectric loss of dielectric polymers filled with high-k coated conducting particles with the objective to aid the design of polymer/filler composite materials with high dielectric constant and low dielectric loss at high frequencies. Our results demonstrate that, contrary to prevailing views, the filler concentration and coating layer thickness dependences of the effective dielectric constant and loss are nonmonotonic. Implications for high-k materials design are also outlined.