The hydrodynamics observed in large-scale gas-solid fluidized bed reactors are different from those observed in smaller scale beds. In this study, the effect of bed diameter on the hydrodynamics of gas-solid fluidized bed reactors has been investigated in two bubbling fluidized beds of 44 cm and 14 cm in diameter using an advanced non-invasive radioactive particle tracking (RPT) technique. Compressed air at room temperature was used as the gas phase, and the solid was glass beads with a particle size of 210 μm (Geldart-B) and density of 2.5 g · cm-3. Particle velocity field, Reynolds stresses, normal stresses, turbulent kinetic energy, and axial and radial eddy diffusivities were measured in two beds at gas velocities of 1.5 Umf, 2 Umf, and 3 Umf. Experimental results showed that the bed scales have a significant effect on some of these hydrodynamic parameters where the magnitude of solids velocity is much higher in the larger bed and the solids mixing and diffusion of particles are increased by increasing the column diameter.