We propose a practical method for the treatment of turbulent mixing rate in a two-phase subchannel flow in a hydrodynamic non-equilibrium state. Based on the assumption that the fundamental modes of the inter-subchannel fluid transfer in such a state are turbulent mixing, void drift, and diversion cross flow, the turbulent mixing rate is considered to be equal to that in the hydrodynamic equilibrium state that the flow will attain. The applicability of the method is examined by experiments concerning the axial variation in tracer concentration in a non-equilibrium flow without diversion cross flow. A good agreement is seen between the calculations and the measurements.