This highly thermally-conductive silicone composition is obtained by blending, as thermally-conductive fillers at a specific ratio and in specific amounts in (A) a silicone composition containing an organopolysiloxane as a main agent, (B) a spherical magnesium oxide powder having an average sphericity of 0.8 or more, an average particle size of 80-150 µm, and a purity of 98 mass% or more, and (C) (C-I) a spherical aluminum oxide powder which has an average sphericity of 0.8 or more and an average particle size of 7-60 µm, and in which the proportion of rough particles of 96-150 µm is 0.1-30 mass% in the entire component (C-I) in a laser diffraction particle size distribution, and (C-II) a spherical or irregularly-shaped aluminum oxide powder having an average particle size of 0.1-4 µm. The thermal conductivity of the composition is 7.0 W/m·K or more, and the viscosity of the composition at 25°C is 30-800 Pa·s. This highly thermally-conductive silicone composition has excellent electrical insulating properties and thermal conductivity.