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, gamma(s/l), between the basal plane of graphite and the Fe-C melts at 1573 K in a purified He-3%H-2 atmosphere. For the calculation of gamma(s/l), we have to use the true graphite surface energy, gamma(s/v) and the surface tension of the iron melt, gamma(l/v), as the reference. However, we could not get a reliable gamma(s/l) value from the literature, because the gamma(s/v), value could be changed by the contamination of the vapor from the melt. For this reason, we measured the gamma(s/v) value using a nonequilibrated method based on the interfacial energy balance at the trijunction using gamma(l/v) and the contact angle theta and phi, where theta is the contact angle between the liquid metal and graphite substrate observed during the measurement by an optical microscope, and 0 is the hidden angle observed at cross section of the sample. We found that the gamma(s/v), and gamma(s/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 gamma(s/l) decreases due to increase in the sulfur content. (C) 2005 Springer Science + Business Media, Inc.