Many Invasive and Non-Invasive Techniques Have Been Used to Analyze the Hydrodynamics of Fluidized Beds. in This Study, the Effect of Superficial Gas Velocity and Bed Particle Density on the Hydrodynamics of Gas–solid Fluidized Beds Was Investigated by using a Cylindrical Plexiglas Fluidized Bed Column, 14 Cm in Diameter. Air at Room Temperature Was Used as the Fluidizing Gas and Two Different Geldart Type-B Particles Were Used: Glass Beads and Copper Particles with Material Densities of 2.5 and 5.3 G/cm3, Respectively, with the Same Size Particle, 210 µm. to Measure the Time-Averaged Cross-Sectional Gas and Solid Holdup Distribution, Gamma Ray Computed Tomography Was Used for the First Time as a Non-Invasive Technique Instead of using X-Rays (Due to the Height Attenuation of the Copper Particles). the Results Show that Gas Holdup Increases by Increasing the Superficial Gas Velocity, and Decreasing the Particle Density Increases the Gas Holdup in the Bed.