The rate of hydrogen desorption in the mold stream degassing process was measured to investigate the effect of hydrogen partial pressure. Experiments were conducted on 10.5 mass% Cr steel, 100 ton scale ingots. The hydrogen desorption reaction was controlled by metal side mass transfer at the gas/metal interface in this experimental condition. And the value of the volumetric mass transfer coefficient was estimated to be approximately 5.18 s−1. As is the case with the hydrogen desorption reaction, the rate of nitrogen desorption was measured, and the nitrogen desorption reaction was controlled by metal side mass transfer. Some mass transfer models such as Film theory, Penetration theory, and Surface renewal theory were taken into account to explain the present results. The mass transfer mechanism of hydrogen and nitrogen desorption in this process is explained by the Penetration theory or Surface renewal theory.