Nitrogen gas absorption phenomena in molten steel during bottom bubbling were experimentally studied. Changes of nitrogen in molten steel were observed with 10, 14 and 200 kg scale experiments and it was confirmed that absorption rate of nitrogen were controlled by mass transfer. In addition, bubble dispersions in each experiments were visualized with volume of fluid method (VOF) solver of OpenFOAM and the area of free surface, AS and bubble, AB were numerically evaluated.The measured data were evaluated with a mixed control model of liquid-phase, gas-phase and chemical reaction. Numerically evaluated AS and AB were utilized for separation from volumetric rate constant of nitrogen absorption, Akm to mass transfer coefficient, km. As a result, mass transfer coefficient at bubble, km,B and free surface, km,S were obtained. In addition, relation between stirring power density, ε ˙ and both km,B and km,S were investigated and it was revealed that km,B and km,S increased in proportion to ε ˙ 0.6.