Heat transfer and fluid flow by turbulent natural convection were studied in a vertical enclosed rectangular container having a high aspect ratio and composed of heated and cooled plates maintained at constant temperature. The working fluids were air for low Rayleigh numbers, and are water and silicone oil of 1 cSt for high Rayleigh number. The velocity field in a vertical enclosed rectangular container was visualized and analyzed by particle image velocimetry (PIV) in order to examine the relation between heat transfer and fluid flow. The experimental results were found to be well correlated with the heat transfer correlation proposed previously.