A heat dissipation substrate having a metallic layer with few defects on its surface is obtained by a process including the steps of: forming a metallic layer by plating on the surface of an alloy composite mainly composed of a powder of a principal metal, additional metal and diamond; and heating and pressurizing alloy composite coated with metallic layer, at a temperature equal to or lower than melting points of the metallic layer and the alloy composite. Consequently a heat dissipation substrate is obtained which has a coefficient of linear expansion of 6.5 ppm/K or higher and 15 ppm/K or lower as well as a degree of thermal conductivity of 420 W/m·K or higher, the substrate having a metallic layer with few defects in its surface layer and thereby allowing for a Ni-based plating on which the void percentage in the solder joint will be 5% or lower.