In order to estimate work of external force per unit area to infiltrate molten aluminum into bundle of two types of carbon fibers which were PAN (polyacrylonitrile) based and pitch based, wettability of graphite by molten pure aluminum and aluminum – 2.0, 2.9 and 9.1 mass % magnesium alloy was measured using sessile drop method. Basal plane and prismatic plane exist on the surface in the case of PAN and pitch based fibers, respectively. The wettability was examined in the atmosphere of Ar + 3 vol. % H2 at 1189 K. Both the contact angle and the surface tension were estimated using Young-Laplace equation. The surface tension decreased form 1.0 N/m at 0 mass % Mg to 0.7 mass % Mg at 9.1 mass % Mg. The contact angle between molten aluminum and basal plane or prismatic plane of graphite was 127º or 148º respectively. In the case of basal plane, the contact angle increased with increasing magnesium content. In the case of prismatic plane, whereas, the contact angle decreased from 155º at 2.0 mass % Mg to 125º at 2.9 mass % Mg. The work of external force for infiltration was calculated using Young’s equation. In the case of basal plane, the work was constant at 0.6 J/m2 regardless of magnesium content. In the case of prismatic plane, the work decreased with increasing magnesium content. It was found that the influence of magnesium content on the work would be different between using PAN-based carbon fiber and using pitch-based carbon fiber.