Temperature distributions of fluid in an agitated tank equipped with a helical ribbon impeller are visualized by means of liquid crystal particles and the relation between temperature distribution and circulation flow is observed. It is concluded that non-uniformities in temperature in an agitated tank should not be neglected in the laminar flow region and that, in this case, a flow model realistic enough to represent the actual flow behavior is necessary. A flow model is developed and applied in order to analyze the steady-state heat transfer system. The introduction of this flow model has been found very effective in expressing the thermal mixing process in an agitated tank with a helical ribbon impeller.