We report the preparation and characterization of thermally conductive and transparent polycarbonate (PC)/alumina (Al2O3) composites. First, nano Al2O3 powder was synthesized by flash pyrolysis process and calculated amount of Al2O3 was dispersed into the methylmethacrylate (MMA) followed by bulk polymerization. PC/poly-(methylmethacrylate) (PMMA)-Al2O3 nanocomposites with various amounts of Al2O3 was prepared by mixing bulk polymerized PMMA-Al2O3 composites with PC through melt-mixing. Incorporation of Al2O3 in the bulk polymerized PMMA resulted in good dispersion of the Al2O3 particles in the PC matrix. The effect of particle size of Al2O3 on thermal conductivity, mechanical and optical properties of the composites was investigated.