Gas-liquid-solid mini-fluidized bed is a new and important reactor. However, the flow behavior in such a system is not well understood, even for the characteristics of single bubble. Initial fluidization, movement and size of singe bubble in three-phase co-current upward mini-fluidized beds of 2-10 mm sizes were studied with visual experiments. The results show that the variation of pressure drop across the bed with time is a strong fluctuation due to the coalescence of bubbles at lower superficial liquid velocities for smaller beds, while no obvious difference in the minimum fluidization velocity between the liquid-solid and gas-liquid-solid systems for a 10 mm MFB exists in the experimental ranges. The wall effect on the bubble size is dominant. The stronger wall effect decreases suspension inertial force, which leads to the diminution of bubble size. A force balance between the surface tension force and buoyant force dominates the bubble size in mini-bubble column at lower orifice gas velocities, while suspension inertial force plays more important role in governing the bubble size in three-phase mini-fluidized beds. Additionally, compressibility of the gas phase contributes to the variation in the bubble size; the bubble wake is observed even for such small solid particles, and can affect the following bubble size in smaller beds at relative high orifice gas velocities. An empirical equation was suggested to predict the bubble diameters in three-phase mini-fluidized beds. (C) 2015 Elsevier B.V. All rights reserved.