The viscosities of the industrial basic oxygen furnace (BOF) slag with varying compositions of MgO, Al2O3, TiO2, and MnO were continuously measured at a temperature range between 1400 and 1700◦C using the rotating bob method. Three characteristic temperatures for the melting behavior of the BOF slag were investigated using a high-temperature microscope. The solid fraction of the slag was calculated by FactSage 7.2 using the FTOxid database. General observations from the experimental data show that the increase in MgO tends to increase viscosity. However, Al2O3, TiO2, and MnO decrease viscosity up to a certain level, and beyond that, they also increase the viscosity. The measured values of the viscosity of BOF slags were compared and discussed with known data from the literature. Finally, the activation energy of BOF slags with different compositions of MgO, Al2O3, TiO2, and MnO was calculated in the temperature range of industrial operations. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.