Hydrodynamic study was performed on the gas-liquid-solid micro-fluidized beds with 0.8 mm bed diameter and multi-bubbles through high speed camera recording system. Flow regimes including dispersed bubble flow, coalesced bubble flow and slug flow were identified from the multi-bubble flow. Wall effect at the small bed-to-particle diameter ratio led to the occurrence of bubble coalescence and flow regime transition at lower solid holdups. Obvious bed contraction appeared only at lower liquid velocities or higher gas velocities. The diameter range of the spherical micro-bubbles in the micro-fluidized beds was 200-500 mu m. The bubble size in dispersed bubble flow was normally distributed and slightly increased with the solid holdup. And in coalesced bubble flow, the bubble size was decided by the overall coalescence probability and presented a special step distribution. The larger coalesced bubbles concentrated the distribution of bubble terminal velocity. And the bubble terminal velocity decreased with the solid holdup and inversely with the bubble size. In dispersed bubble flow regime, there was positive linear relationship between the apparent viscosity and the solid holdup. Three-phase micro-fluidized beds meet the further development of micro-reactors, and this study on the hydrodynamic characteristics contributes to their wider application. (C) 2017 Elsevier B.V. All rights reserved.