In the Powder Bed Fusion-Laser Beam(PBF-LB) additive manufacturing, a laser beam is applied to a thin powder layer to selectively melt the powder to produce a melt pool of a semi-spherical shape. However, this localized laser heating causes a temperature gradient by melting and a concentration gradient upon solidification. These temperature and concentration gradients may result in the gradient of surface tension, yielding Marangoni flow at the surface of the melt pool. The Marangoni flow causes defects in the final products. Therefore, it is essential to gain knowledge of the surface tension of the metal powders as a function of temperature and concentration. In this study, the surface tension of liquid Fe-Si alloys was investigated with an advanced surface tension measurement technique: the constrained drop method. With the help of a laser backlight and a specially designed alumina crucible, an accurate surface profile of an axi-symetric liquid metal could be obtained. The information on the surface profile was used to solve the surface tension according to the Laplace equation with lab-made software. The addition of phosphorus was helpful in minimizing the temperature coefficient of the surface tension. It was expected that the present methodology would be very powerful in designing the chemical composition of metallic alloy powders used in PBF-LB additive manufacturing.