This paper reported the observation of inclusions in the steel of an ingot runner. Thermodynamic modeling was performed to study the formation mechanism and sources of inclusions. CFD modeling on fluid flow, heat transfer, solidification of molten steel was carried out and the entrapment of inclusions in runner steel was predicted and compared with the measurement. Al2O3-based inclusions were the main ones in the steel samples. MgO?Al2O3 mainly stemmed from the lining refractory. The high MnO inclusions in the runner steel came from the runner slag. The effect of natural convection was very important for the cooling and solidification of the steel in the runner. Both the observation and the modeling show that > 50μm inclusions more accumulated on the upper area in the runner and < 50μm inclusions dispersed well and more accumulated on the lower area of the runner. This paper reported the observation of inclusions in the steel of an ingot runner. Thermodynamic modeling was performed to study the formation mechanism and sources of inclusions. CFD modeling on fluid flow, heat transfer, solidification of molten steel was carried out and the entrapment of inclusions in runner steel was predicted and compared with the measurement. Al2O3-based inclusions were the main ones in the steel samples. MgO? Al2O3 mainly stemmed from the lining refractory. The high MnO inclusions in the runner steel came from the runner slag. The effect of natural convection was very important for the cooling and solidification of the steel in the runner. Both the observation and the modeling show that> 50μm inclusions more accumulated on the upper area in the runner and <50μm inclusions dispersed well and more accumulated on the lower area of ​​the runner.