To improve the thermal conductivity of epoxy resin without losing the insulation, epoxy/silicon nitride (Si₃N₄) nanowire composites were produced. β-Si₃N₄ nanowires were synthesized by the carbothermal reduction and nitridation of a homogeneous mixture of silica (SiO₂), carbon, and a small amount of cobalt via the vapor−liquid−solid (VLS) mechanism. The ratio of β to α phase in the product Si₃N₄ increased with heat-treatment temperature; however, the undesirable SiC also increased. By increasing the nitrogen gas pressure in the heat treatment, the ratio of β- to α-Si₃N₄ increased without producing SiC. The epoxy composite containing Si₃N4 nanowires of 60 vol.% showed high thermal conductivity of 9.2 Wm^-1K^-1 along the preferred orientation of the nanowires.