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We used an improved sessile drop method, which can measure the equilibrium contact angle of molten Fe-C-S alloys on graphite basal planes using an equilibrated method. We adopted this method for the measurement of the interfacial energy, γs/l, between the basal plane of graphite and the Fe-C melts at 1573 K in a purified He-3%H2 atmosphere. For the calculation of γs/l, we have to use the true graphite surface energy, γs/v and the surface tension of the iron melt, γl/v, as the reference. However, we could not get a reliable γs/l value from the literature, because the γs/v value could be changed by the contamination of the vapor from the melt. For this reason, we measured the γs/v value using a nonequilibrated method based on the interfacial energy balance at the trijunction using γl/v and the contact angle θ and φ, where θ is the contact angle between the liquid metal and graphite substrate observed during the measurement by an optical microscope, and φ is the hidden angle observed at cross section of the sample.We found that the γs/v and γs/l values decrease when the sulfur contents are greater than 10 mass ppm sulfur. From 10 to 120 ppm sulfur, the decreasing rate has a constant value on a logarithmic scale of sulfur activities. The morphology of the graphite changes from spheroidal to flake as γs/l decreases due to increase in the sulfur content.