Hydrodynamic similarity in the fully developed zone of co-current upward gas-solid two-phase flow systems under different operating conditions was investigated by measuring the axial profiles of pressure gradient, radial profiles of solid concentration and particle velocity in two circulating fluidized bed (CFB) risers of 15.1 and 10.5 m high, with FCC and sand particles, respectively. The experimental data obtained from this work and in the literature show that when the scaling parameter. G(s)/(rho U-p(g)), is modified as Fr(D)(-0.3)G(s)/(rho U-p(g)), a detailed hydrodynamic similitude of the gas-solid flow in the fully developed zone of the risers under different operating conditions can be achieved. Furthermore, the experimental results from different gas-solid flow systems also show that as long as Fr(D)(-0.3)G(s)/(rho U-p(g)) remains constant, there is the same solid concentration in the fully developed zone of different CFB risers with different particles. With the same 0.3. the local solid concentrations, the descending particle velocities, the cluster frequencies and the solid concentrations inside clusters in the fully developed zone of the risers all display the same axial and radial distribution, respectively. In other words, the empirical similarity parameter, Fr(D)(-0.3)G(s)/(rho U-p(g)), appears to have incorporated the effects of operating parameters (G(s) and U-g), so that, the gas-solid flow in the fully developed zone of CFB risers under those different operating conditions but having the same Fr(D)(-0.3)G(s)/(rho U-p(g)), shows similar micro- and macro-hydrodynamic characteristics. The study shows that the empirical similarity parameter, Fr(D)(-0.3)G(s)/(rho U-p(g)), is also independent of the upward gas-solid flow systems. Crown Copyright (C) 2008 Published by Elsevier Ltd. All rights reserved.