Anaerobic reactor is one of gas-liquid-solid three-phase bio-reactors and has attracted high attentions worldwide due to biogas, a renewable energy. As is well-known, hydrodynamic conditions play crucial roles in the performance of the three-phase reactors. But the knowledge about the hydrodynamic conditions around sludge (microbial aggregates) is limited. The hydrodynamic shear force exerted on sludge derives from the liquid flow and rising biogas bubbles nearby. The in situ investigation in a two-dimensional micro anaerobic reactor shows that 56.8-96.6% shear rate exerted on one piece of granular sludge (major axis of 2 mm and minor axis of 1.5 mm) in typical motions stems from the rising bubbles when biogas production rate is greater than 0.1 m(3) kgVSS(-1) d(-1), and the superficial liquid velocity of reactor was fixed at 6.48 mh(-1) in this study. Furthermore it linearly correlates with specific bubble population with R-2 of superior to 0.95. The specific bubble population plays more important roles in the shear rate on granules than the bubble diameter. Liquid flow is also important for the shear rate exerted on moving sludge in term of the relative velocity between sludge and liquid flow rather than the superficial liquid velocity. Thus the shear rate derived from liquid flow would be significantly lower than that part originated from bubbles. A high-speed digital camera and a particle image velocimetry (PIV) system were used to in situ quantify the bubbles and their behavior in reactor. This study could facilitate understanding and improving the hydrodynamic conditions in three-phase bio-reactors. (C) 2015 Elsevier B.V. All rights reserved.