Thermophysical properties of undercooled liquid monotectic alloys are usually difficult to be determined because of the great difficulty in achieving large undercoolings. We measure the surface tension of liquid Fe_77.5Cu₁₃Mo_9.5 monotectic alloy by an electromagnetic oscillating drop method over a wide temperature range from 1577 to 1784 K, including both superheated and undercooled states. A good linear relationship exists between the surface tension and temperature. The surface tension value is 1.588 N/m at the monotectic temperature of 1703 K, and its temperature coefficient is −3.7×10⁻⁴ Nm⁻¹K⁻¹. Based on the Butler equation, the surface tension is also calculated theoretically. The experimental and calculated results indicate that the effect of the enriched element on droplet surface is much more conspicuous than the other elements to decrease the surface tension.