The reduction of MnO in molten slag with carbon undersaturated iron was studied.It was found that the process is affected by the carbon content of molten metal and the temperature.The higher the carbon content and the temperature,the faster both the reduction and the emerging of the hump on curve of ω_(FeO),the larger the difference between ω_(FeO,max) andω_(FeO,e).The phenomena were explained with three-step reaction model. The reduction of MnO in molten slag with carbon undersaturated iron was studied. It was found that the process is affected by the carbon content of molten metal and the temperature. Higher than carbon content and the temperature, the faster both the reduction and the emerging. of the hump on curve of ω_ (FeO), the larger the difference between ω_ (FeO, max) and ω_ (FeO, e). The phenomena were explained with three-step reaction model.