The bubble characteristics for multi-phase flow systems were studied in a two-dimensional column by direct visual observation together with the image processing of photographs. The effects of operating conditions such as particle sizes, superficial gas and liquid velocities on bubble flow behavior were examined. Bubble size distributions in both two- and three-phase systems operated both semi-batchwise and continuously were found to be fairly well represented by the log-normal distribution. The variation in bubble flow behavior appears to be closely related to the difference in fluidized states of solid particles. In small particle systems (gas-sparged slurry reactors), the bubble flow behavior is similar to that in two-phase systems. On the other hand, larger particle systems (three-phase bubble columns and three-phase fluidized beds) exhibit appreciably different bubble behavior and show the dependency of the gas velocity on the Sauter mean diameter of bubbles and the geometric standard deviation of bubble size distribution.