As an advanced electronic packaging material, SiCp/Al-20Si composites have been fabricated by high pressure solidification. The mechanism of formation of fibrous eutectic Si and the thermal conductivity (TC) behavior of the composites have been systematically investigated. High pressure decreases diffusion of Si phase significantly; moreover, SiC addition hinders the diffusion and provides nucleation sites for the Si phase. Both high pressure and SiC contribute to the formation of fibrous eutectic Si. The SiCp/Al-20Si composite solidified at 3 GPa exhibits lowest TC due to the formation of a supersaturated Al(Si) solid solution, whereas it is highest after ageing treatment. The optimal TC of the 45% SiCp/Al-20Si composite is 166.8 W/m·K at room temperature. The TC of the composites can be well predicted by the Hasselman-Johnson (H-J) theoretical model.