Heat transfer by turbulent natural convection in a cubical enclosed rectangular container composed of heated and cooled side plates maintained at constant temperatures was studied using Particle Image Velocimetry (PTV), Laser Induced Fluorescence (LIE) and Large Eddy Simulation (I,ES). Water was chosen as the working fluid in order to achieve high Rayleigh number [low. The velocity field and temperature field in the container were experimentally visualized and analyzed by PIN and LIE, respectively, in order to examine the relationship between heat transfer and fluid flow Numerical simulation was conducted with [ES in order to validate the experimental results. The heat transfer coefficient obtained from the experimental results was found to be consistent with the heat transfer correlations proposed previously. Heat transfer coefficient and fluid flow obtained from the numerical simulation were found to coincide well with the experimental results.