The aim of this study is to investigate the effect of gas flow velocity, size of sand particles, and the distance between tubes immersed in a fluidized bed on heat transfer coefficient. Experimental tests were conducted on a bundle of copper tubes of (12.5 mm) diameter and (320 mm) length arranged in a matrix (17×9) and immersed in a fluidized bed inside a plastic container. One of the tubes was used as a hot tube with a capacity of (122 W). (25 kg) of sand with three different diameters of sand particles (0.15, 0.3 and 0.6 mm) was used in these tests at ten speeds for gas flow (from 0.16 m/s to 0.516 m/s). The results showed a significant inverse effect of fluidized bed particles diameter on the heat transfer coefficient. Accordingly, the heat transfer coefficient for (0.15mm) diameter sand was found to be higher than that of (0.3 mm) and (0.6 mm) sand by about (3.124) and (6.868) times respectively, in all tests. The results showed good agreement with results from other studies conducted under the same conditions but with different sand particle size.