Aerobic granular sludge has been recognized to be promising for wastewater treatment. Their hydrodynamic characteristics have a significant impact on the mass transfer process in reactors. In this study, the hydrodynamic characteristics of aerobic granules were studied using an experimental approach, and their fluid dynamic behaviors were analyzed using a numerical approach. Experimental results show that the aerobic granules are fractal-like aggregates with porosity. Their porosity and permeability were found to increase with increasing granule size. The numerical model simulated the flow field surrounding a granule, which distinguished the flow behaviors of the granules with different permeability at different outflow Reynolds numbers. The velocity vectors colored by velocity magnitude in the granule internal depended significantly on the permeability of the granule and the Reynolds number. The results provided a helpful tool to investigate the hydrodynamic behavior of aerobic granules with a consideration of their porous structure characteristics. (C) 2010 American Institute of Chemical Engineers AIChE J, 57: 2909-2916, 2011