Slag foaming measurements were carried out for BOF type slags that exist during the first half of the blowing period in order to better understand slopping of slag. The foam index (Σ) decreases with increasing FeO up to about 20% FeO content and is almost constant for FeO from 20 to 32 % FeO. This is believed tobe because above about 25% FeO, the viscosity is nearly constant. The foam index shows a minimum value at basicity[5(CaO1MgO)/(SiO₂+Al₂O₃ )] of 1.4 at 1 713 K; it increases at higher basicities due to the precipitates such as 2CaOSiO₂ or (Fe, Mg)O which stabilizes the foam. The effect of TiO₂ and MgO on foam index was also evaluated. An empirical equation for the foam index obtained by the previous researchers was applied to the present experimental results. The foaming during the first half of the blowing in BOF process was described based on the empirical relationship and the foam height was estimated for a BOS converter as a function of decarburization rate.