Abstract: The interaction between gas and particles under different flow regimes, ranging from minimum fluidization to pneumatic transportation, was investigated in a vertical gas-solid flow system. It is found that the drag coefficients could be used to account for the energy exchange in the two-phase flow system. Two diagrams of the drag coefficient at different operating conditions are also obtained. The results of this work show that the heterogeneity of the gas-solid flow (e. g., the bubbling and particle clustering phenomena)could be considered as. the results of an optimal arrangement in which the interaction between gas and particles approaches minimum.