The complexity of flow in an agitated tank comes in part from the global change of flow pattern from one type to another alternately on a long time scale. This phenomenon is known as macro-instability (MI). In this work, it was characterized by means of flow visualization experiment and confirmed with computational fluid dynamics (CFD) simulations. The results showed various combinations of flow patterns occur-ring repeatedly in the whole vessel for all configurations observed. The flow patterns inside agitated tank equipped with single impeller arrangement could be classified into the radial discharge and cross-pass types for agitation with a Rushton turbine (RT) impeller, radial discharge and axial discharge for a fan turbine (FT) impeller, axis symmetric and asymmetric axial discharges for a pitch blade turbine (PBT) impeller. In case of double impeller arrangement, there were more complex variations of flow patterns due to the interaction between the flow patterns produced by the upper and lower impellers. The frequency of appearance and life time ratio of each recognized flow type was also observed in experimental and simulation works. According to the agreement between the experimental visualization and simulation in both flow pattern recognition and its characteristic, the combination of large eddy simulation (LES) and sliding mesh (SM) method is suitable for the prediction of MI phenomenon in stirred tank. (C) 2009 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.