Predictive models for describing the hydrodynamic behavior (bed-expansion and bedpressure gradient) of a three-phase anaerobic fluidized bed reactor (AFBR) was developed according to wake theory together with more realistic dynamic bed-expansion experiments (with and without internal biogas production). A reliable correlation equation for the parameter k (mean volume ratio of wakes to bubbles) was also established, which is of help in estimating liquid hold up of fluidized beds. The experimental expansion ratio of threephase fluidized beds (EGLS) was approximately 18% higher than that of two-phase fluidizedbeds (ELS); whereas the experimental bed-pressure gradient of the former [(-△P/H)GLS] was approximately 9.3% lower than that of the latter [(-△P/H)LS]. Both the experimental andmodeling results indicated that a higher superficial gas velocity (ug) gave a higher EGLS and a higher EGLS to ELS ratio as well as a lower (-△P/H)GLS and a lower (-△P/H)GLS to (-△P/H)LS ratio. As for the operation stability of the AFBR, the sensitivity of ug to expansion height (HGLS) and (-△P/H)GLS is between the sensitivity of superficial liquid velocity and biofilm thickness. The model predictions of EGLS, (-△P)GLS, and (-△P/H)GLS agreed well the experimental measurements. Accordingly, the predictive models accounting for internal biogas production described fairly well the hydrodynamic behavior of the AFBR.