A fast-scanning X-ray CT system, designed for measuring dynamic events in multiphase fluids, was used to obtain the transient 2-dimensional cross sectional gas-phase distribution in a fluidized bed. The captured data was sent to a PC for image reconstruction, and the density distributions across the column cross section were visualized, Using image processing techniques, a pseudo 3-dimensional image of bubbles was obtained. The spatial and time resolutions of the system were sufficient for the measurement of ascending bubble behavior in a fluidized bed with fine particles. Bubble shape, interfacial area, and bubble holdup over the cross section can be measured by this system.