The density of liquid Fe-S was measured at 4 GPa and 1,923 K using a sink/float method with a composite density marker. The density marker consisted of a Pt rod core and an Al(2)O(3) tube surrounding. The uncertainty in the density of the composite marker is much smaller than that of the composite sphere, which had been used in previous density measurements. The density of liquid Fe-S decreases nonlinearly with increasing sulfur content at 4 GPa and 1,923 K. This tendency is consistent with the results measured at ambient pressure. The molar volume of FeS calculated from the measured density gradually increases with sulfur content. The excess molar volume from ideal mixing of Fe and S at 4 GPa was negative value. The new method proposed here is applicable to the density measurement of other Fe alloys at high pressure. The tendency of the molar volume and the excess molar volume with sulfur content at ambient pressure is consistent with these at high pressure at least up to 4 GPa. The excess molar volume at high pressure is essential for estimating the amount of light elements in the outer core.