It is important to properly understand and calculate the distributions of the heat flux and the heat transfer coefficient in a furnace for designing a circulating fluidized bed (CFB) boiler, especially for one with supercritical parameters. An experimental study of the heat transfer in a commercial 300 MWe CFB boiler was conducted in this work. The heat flux from the bed to the waterwall was measured by a heat flux meter at four different heights. The influence of suspension density and bed temperature was analyzed by measuring the pressure profile and temperature profile in the furnace. In addition a numerical modeling study on the two phase flow in the furnace was carried out. The results indicate that the density of the solids suspension was non-uniform in the cross section at a certain height, and decreased gradually with increasing furnace height. Consequently, the distribution of heat flux in the horizontal plane was non-uniform and decreased along the height of the furnace. The heat flux showed a decreasing trend near the center. (C) 2010 Elsevier B.V. All rights reserved.