The paper reports the study of hydrodynamic behavior of two phase flow (water-air) in a real membrane bio-reactor (MBR) system. At actual working condition, there are massive air bubbles with different sizes distributed in the MBR system. Using particle image velocimetry (PIV) technique, in order to block the strong light reflection from big bubbles and avoid measurement errors induced by small bubbles, fluorescent particles and red filter lens are used in the measurement. A high-speed camera is also used to examine the bubble behavior inside the MBR system. The measurement results include water velocity of single phase and two phase flows, and bubble size distribution and movement. It is observed that single phase flow, whether it is water flow or air flow, will not cause the obvious vibration of fiber bundle; only the combination of water flow plus aeration will cause the heavy oscillation of fiber bundle. Water goes upward along the vertical center line and flows downward along the side walls of the water tank for both single phase and two phase flows. The size range of air bubbles varies from less than 1 mm to around 50 mm, and a great majority of air bubbles have the diameter of 3-5 mm. Air bubbles vertically go up in the lower part of MBR unit, and break into small bubbles or combine into bigger bubbles in the top part of MBR unit due to heavy oscillation of water and pressure changes. The measurement results are used to optimize the CFD model by adopting a variable bubble size, which will reduce the error induced by assuming constant bubble size and improve the accuracy of result predictions. (C) 2010 Elsevier B.V. All rights reserved.