In this work, a detailed investigation on the effects of particle diameter on the fluidization characteristics of gas–particle flows inside a bubbling gas–solid fluidized bed has been carried out. The studies are performed using an in-house flow solver developed employing the Eulerian–Eulerian two-fluid model. Simulations carried out using the solver indicate that particle diameter has significant effects on the overall flow hydrodynamics inside a bubbling gas–solid fluidized bed. Numerical evidence suggests that time-averaged particle velocity at any section inside the fluidization zone decreases, while the time-averaged particle volume fraction increases with an increase in particle diameter. For a given diameter, the magnitude of maximum time-averaged particle velocity at any section has been found to increase gradually inside the fluidization zone as we move toward the outlet. Interestingly, this variation in the time-averaged particle velocity with height becomes less prominent with the increase in particle diameter. © 2019, OWZ.