The interface area between the bubble and emulsion phases in a fluidized catalyst bed is one of the important parameters used to analyze and design the fluidized bed reactor. We used a fast-scanning X-ray CT system to observe the bubble shape and structure. We then obtained the transient 2-dimensional cross sectional gas-phase distribution in a fluidized catalyst bed. Using image-processing techniques, pseudo 3-dimensional images of the bubbles were reconstructed. The bubble structure was studied based on the 3-dimensional images and the previously obtained results in a 2-dimensional fluidized bed. It was found that the bubble shape was not spherical but complicated, and that the bubbles ascending in a fluidized catalyst bed consisted of some smaller bubbles.