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This paper deals with the thermal design of an electronics package and a demonstration of reduced thermal resistance for high-power amplifiers (HPAs). The focus is package internal thermal management. A carbon fiber-reinforced carbon composite- (C/C composite) based heat sink is proposed as a means of enhancement over the more conventional CuMo material. The C/C composite has anisotropic thermal properties. Thermal performance of the material with anisotropic thermal properties depends strongly on taking advantage of superior properties in the desired directions. Finite-element analysis is performed to determine the correct orientation of the C/C composite material with anisotropic thermal conductivities to minimize thermal resistance. A 32% reduction in thermal resistance of the HPA has been predicted in the initial simulation. A package incorporating the C/C composite material is built with the optimal orientation of thermal anisotropy obtained by numerical simulations. A 20% reduction in thermal resistance has been successfully obtained by surface temperature measurements for the HPA with the C/C composite material. The difference between numerical (32%) and experimental results (20%) is well explained by the difference in boundary conditions at the package base. Also, nonlinearity in thermal resistance is explained by taking account of temperature dependence of semiconductor materials, such as SiC and GaN.