In order to have good dephosphorization, it is beneficial to know the proper physical properties, including the melting point and viscosity of slag with fixed lime activity for steel refining, slag foaming and reaction in steel making. A fast way to determine the melting point and viscosity of complex multi-component slags is needed to control the quality of steel. In this study, experimental slag of FetO-CaO-SiO2-MgO-MnO-Al2O3 is obtained with electro slag remelting technique. The liquidus temperature of slag is determined with a video-based contact angle meter recorded with a high-speed camera and verified with an isopleth phase diagram of slag constructed with thermodynamic software FACTSAGE. The viscosity of liquid slag can be estimated with the optical basicity and liquidus temperature obtained from the calculated isopleth phase diagram, then it was compared with the experimental value measured by the rotating torque viscometer. The results indicated that substantial error between the calculated temperature of isopleth phase diagram and the value determined with method of contact angle in high basicity slag. On the other hand, minor difference exists between calculated liquidus temperature and measured temperature in low basicity slag. The viscosity estimated from the optical basicity and liquidus temperature of isopleth phase diagram show the similar behavior. It has good correlation between the calculated value and experimental value in low basicity, however, deviations are found in the range of high basicity.