Investigation in the present paper has been made theoretically and experimentally on a development of the Karman vortex street due to the natural convection in an opposing flow. Unsteady numerical solutions are obtained by means of a time-dependent numerical analysis. A development of the Karman vortex street due to the natural convection is predicted based on the result of numerical analysis. To verify the validity of the prediction, flow visualizations are carried out. The following results are obtained.1. A development of the Karman vortex street is predicted by means of a numerical analysis.2. The validity of the development is verified by means of flow visualizations.3. The mechanism of the development is made clear by the simultaneous measurements of velocity and temperature.