Hydrodynamics plays an important role in the design and operation of bioreactors for wastewater treatment. In this work, the response of anodic biofilm to hydrodynamic shear in a common two-chamber bioelectrochemical system (BES) was investigated. The instantaneous flow field in the anodic compartment of the BES was obtained through particle image velocimetry. The effect of anodic hydrodynamic shear on the BES performance in terms of current density, coulombic efficiency and internal resistance was evaluated successively. In addition, the microbial communities of anodic biofilm that alter with hydrodynamic changes were revealed. The results show that the flow regime involving with vortex and dead zone in the anode compartment of the BES was complicated. An optimum hydrodynamic shear existed to yield the maximum stable current density and coulombic efficiency as well as the lowest anodic internal resistance for the BES. Moreover, the microbial communities of the anodic biofilm were sensitive to the hydrodynamic shear. The largest biomass of the biofilm attached on the electrode surface and the highest relative abundance of electrochemically active bacterium, such as dominant Desulfovibrio and Geobacter, were observed under the optimum hydrodynamic conditions. (C) 2017 Elsevier Ltd. All rights reserved.