A process for producing a substrate, which comprises processing an aluminum/graphite composite into plates having a thickness of 0.5-3 mm using a multi-wire saw under the following conditions (1) to (4): (1) the wires have abrasive grains bonded thereto which are one or more substances selected from diamond, C-BN, silicon carbide, and alumina and have an average particle diameter of 10-100 µm; (2) the wires have a diameter of 0.1-0.3 mm; (3) the wires are run at a rate of 100-700 m/min; and (4) the composite is cut at a rate of 0.1-2 mm/min. The aluminum/graphite composite has a surface roughness (Ra) of 0.1-3 µm, a thermal conductivity at 25 °C of 150-300 W/mK, a ratio of the maximum to the minimum value of thermal conductivity in three perpendicular directions of 1-1.3, a coefficient of thermal expansion at 25-150 °C of 4 × 10 -6 to 7.5 × 10 -6 /K, a ratio of the maximum to the minimum value of coefficient of thermal expansion in three perpendicular directions of 1-1.3, and a three-point bending strength of 50-150 MPa.