The pendant drop method has been developed to measure the surface tension of liquid slag in the static and dynamic conditions by controlling the size of the pendant drop with the aid of Pt-Rh (20%) coil in contact with the slag. Surface tension in dynamic condition, that is, during the rapid change of the drop size, shows a tendency to differ from that in the static condition. Surface tension in the static condition and the temperature coefficient of the surface tension were determined for the liquid CaO-SiO₂, CaO-Al₂O₃, and CaO-Al₂O₃-SiO₂ slags in the temperature range from the vicinity of liquidus temperature to 1873 K. SiO₂ lowers the surface tension while it raises the temperature coefficient of the surface tension from negative at lower SiO₂ contents to positive at higher SiO₂ contents of CaO-SiO₂ and CaO-Al₂O₃-SiO₂ slags. At constant SiO₂ contents of CaO-Al₂O₃-SiO₂ slag, Al₂O₃ lowers the surface tension except for a narrow range of high SiO₂ contents and low Al₂O₃ contents, while it raises the temperature coefficient of surface tension except for the range of high SiO₂ content. The surface tension vs SiO₂ content curve shows a tendency to have a hump in the vicinity of wollastonite and gehlenite for the CaO-SiO₂ and CaO-Al₂O₃-SiO₂ slags respectively.