Heat flow phenomena in the heater unit of an automobile air conditioning system have been analyzed by direct simulation, solving the Navier-Stokes' equation in which an upwind finite difference method of third-order accuracy is adopted with a 2-D model. The analytical results presented visually to ascertain air and heat flows in the heater unit. In addition, a comparative experiment was carried out to compare test results with calculated results.The heater unit in the air conditioning system controls the temperature of air in the automobile. The heater unit consists of a unit inlet, outlet, mixing door and heater, as shown in Fig. 1 (1).Air, entering the heater unit inlet is mixed with warm air heated by the heater to a suitable temperature, and then blown out of the outlet.The shape and size of the heater unit are restricted by the size of an automobile's engine compartment and the layout of other auxiliary devices. Therefore, warm air must be mixed with cool air efficiently in this limited space. The shape of the heater unit has been determined based on experience. The development of the heater unit requires a fairly long period of time and much labor.To improve the efficiency of this development process, the authors quantitatively analyzed air mixing phenomena in the heater unit. The Navier-Stokes' equation was solved directly by the third-order accuracy upwind method.