This paper presents a numerical simulation and a laser holographic interferometric study on thermal diffusion characteristics of combined forced and free heat convection in a counterflow with different temperatures. The purpose of this paper is to make clear the characteristics of a direct mixing heat exchanger for advanced energy conversion. In the numerical study, the Bow patterns and isotherms for both adiabatic and non-adiabatic wall conditions are simulated in two-dimensional fields over a wide range of the Reynolds number and Grashof number for steady and unsteady, laminar and turbulent hows. Real time holographic interferometry is applied to visualize the isotherms for the simulated flow fields and to build a new concept of practical heat exchanger design. In evaluating non stationary interferograms, the time averaged heat flow rate is introduced and the thermal diffusion effect is discussed. As the result, thermal diffusion at the mixing interface of counterflows can be well characterized.