Flow structures induced by bubbles formed in three-dimensional shallow rectangular gas-fluidized beds are investigated by the DEM-CFD Coupling simulation. A numerical code is parallelized and more than 4.5million particles are tracked in the maximum by using 16 CPUs. To investigate the domain-size dependency, the cross-sectional area of beds is enlarged by the factors up to 8 2 and the results are compared. Under the ideal uniform inflow condition, multiple bubbles are spontaneously formed in large cross-section cases. The dependency of bubble size on the cross-section size is not so clear under the same bed height condition. Clear circulation patterns are observed in the time-averaged data. Down-flows of particles exist in low void fraction portions selectively with particle Up-flows dominating in the high void fraction regions. Three-dimensional visualization of bubble shape is also conducted. Bubbles that have complex structures Such as worm-like one exist in addition to the conventional spherical cap. (C) 2007 Elsevier B.V. All rights reserved.