The hydrodynamic characteristics of upflow anaerobic sludge blanket (UASB) reactors were investigated in this study. A UASB reactor was visualized as being set-up of a number of continuously stirred tank reactors (CSTRs) it? series. An increasing-sized CSTRs (ISC) model was developed to describe the hydrodynamics of such a bioreactor. The gradually increasing tank size in the ISC model implies that the dispersion coefficient decreased along the axial of the UASB reactor and that its hydrodynamic behavior was basically dispersion-controlled. Experimental results from both laboratory-scale H-2-producing and full-scale CH4-producing UASB reactors were used to validate this model. Simulation results demonstrate that the ISC model was better than the other models in describing the hydrodynamics of the UASB reactors. Moreover, a three-dimensional computational fluid dynamics (CFD) simulation was performed with an Eulerian-Eulerian three-phase-fluid approach to visualize the phase holdup and to explore the flow patterns in UASB reactors. The results front the CFD simulation were comparable with those of the ISC model predictions in terms of the flow patterns and dead zone fractions. The simulation results about the flow field further confirm the discontinuity in the mixing behaviors throughout a UASB reactor. (D 2008 American Institute of Chemical Engineers AIChE J, 55: 516-528, 2009