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Density of liquid iron alloy under high pressure is important to constrain the amount of light elements in the Earth’s core. Density measurement of solid and liquid Fe₃C was performed using X‐ray absorption image technique up to 9.5 GPa and 1973 K. Density of liquid Fe₃C increases from 6.94 g/cm³ to 7.38 g/cm³ with a pressure of 3.6–9.5 GPa at 1973 K. The bulk modulus of liquid Fe₃C is obtained to be 50 ± 7 GPa at 1973 K. The effect of carbon on the compressibility of liquid iron is similar to that of sulfur, which significantly decreases the bulk modulus of liquid iron. Since carbon dissolution into liquid iron causes reduction of ρ and K_0T, carbon could be excluded from the candidates of alloying light elements in the Earth’s outer core.