The effect of the particle size (dp) and the fluidized velocity investigated Experimentally andtheoretically for different values of heat flux thermal behavior in gas-solid fluidized bed was donewith time. In this work three different particle size was employed (450,650 and 850mµ ). The fluidizingagent was air at different velocities in the range of (2-2.8 m/s). The rig provided with a horizontal heating tube with outer diameter of (3.175cm) was heated eclectically with different power supplies(80,240,350 Watt). Presented mathematical model one dimensional unsteady includes energy, continuity, andmomentum equations for each of two phases. In order to solve the foregoing equations in anumerical way, the discretization method based on control volume procedure has been used thecomputer code (FLBD) which is written with mathlab 2008. The results show that the temperature distribution along the packing height decreases withincrease particle size and the heat flux represents as the temperature increases as the air velocitiesincrease with time. Compared the experimental and theoretical results and the comparison is a goodagreement.