Hydraulic characteristics in a membrane bioreactor are of significance for retarding membrane fouling and maintaining stable operation. Cross flow velocity along the membrane surface and its effect on transmembrane pressure change were experimentally investigated using two pilot-scale submerged membrane bioreactors. A simulation model for calculating cross flow velocity was then developed. Cross flow velocity was a function of aeration intensity. Under the experimental conditions of the present study, there was a critical cross flow velocity of ~ 0.3 m s-1. When the cross flow velocity was lower, transmembrane pressure sharply increased. Predicted cross flow velocities using the simulation model derived from energy balance analysis agreed with the measured values very closely. Cross flow velocity is also dependent on the dimensional parameters of bioreactor. For a reactor that has a greater height, a more compact riser and wider channels in the down-comer and bottom, a higher cross flow velocity could be obtained with the same aeration intensity.