Abstract: The formation of non-isothermal flow in a tundish was investigated in a special designed 2D water modelling system. The critical condition of the formation of non-isothermal flow was proposed to be a number Zb evaluating the sensitivity of tundish flow to temperature difference. CFD simulation of the tracing pattern in a water model described in the initial formation condition of non-isothermal flow. A direct measurement of temperature stratification in a 40 t tundish confirmed the existence of the non-isothermal flow.