Abstract: Similarity of upward gas-solid two-phase flow behavior in the fully developed zone under different operating conditions was further studied based on the axial pressure gradients and local solids holdups from the literature and the authors’laboratory.The results showed that the detailed hydrodynamic similitude in the fully developed zone of the CFB risers under different operating conditions could be characterized by the empirical similarity number presented in this work.It was also found that the solids holdups in the fully developed zone change linearly with the empirical similarity number.For the upward gas-solid two-phase flow systems, operating at different conditions,if the similarity numbers were the same,the local solids holdups,the solids concentrations inside cluster and the cluster existence times would display the same axial and radial distributions, respectively,in the fully developed zone.The preliminary study showed that the empirical similarity number is independent of the upward gas-solid CFB systems.