Aluminum-silicon carbide composite heat spreaders having high thermal conductivity were successfully fabricated by a hot-forging technique, and the effect of an interfacial reaction between Al and SiC on the thermal conductivity of the composites was quantified by X-ray diffraction and theoretical calculations. Al-SiC composites with SiC dispersions were fabricated by forging pure Al-SiC powder compacts at a variety of temperatures, under a high pressure of 900MPa. At forging temperatures below the melting point of Al, porous specimens were obtained, resulting in low thermal conductivity. In contrast, fully-densified specimens having high thermal conductivity were obtained at forging temperatures above the melting point. The interfacial reaction between the Al matrix and the SiC particles produced an interfacial reaction layer of Al4C3, the thickness of which increased with increasing forging temperature. Calculation of the theoretical thermal conductivities of the composites using the Hasselman's model suggests that the increased forging temperature decreases the thermal barrier conductance of the Al-SiC interface, decreasing the overall thermal conductivity of the Al-SiC composites.