Abstract: The evaporation dynamics of trace elements in nickel chrominium molybydenum steel during vacuum induction melting have been studied. The way to determine the transport coefficient of gas boundary layer has been developed. The formula to calculate the specific rate constant of element evaporating from the gas/metal surface has been deduced. The results of experiment and calculation show that Sn or As evaporation process during vacuum induction melting of steel is controlled by transport in both the metal boundary layer and the evaporation from the metal surface, in which K₂₃ value is at the level of 10^-3~10^-2cm/s. The Sn or As loss reaction from the gas/metal surface includes probably the evaporations of itself and its oxide.