The structure of isolated mixing regions was observed in a stirred vessel with the aid of flow-visualization technique. Two toroidal regions were clearly formed respectively above and below the turbine impeller in the range less than Re = 100, Under the condition of this experiment, these regions remained visible for a couple of hours. Under the nearly same rotational condition, a set of three stable filaments surrounding the torus was found in the case of the six-bladed turbine, while a set of four filaments was found in the case of the four-bladed turbine. The velocity fluctuation of now caused by the rotating turbine blades can be regarded as periodical perturbations for the secondary circulating flow. Hence it can be considered that these structures depend on periodical perturbations caused by the rotating turbine blades. In order to observe the inside structure of ring-doughnut-shaped core regions, the unsteady rotation procedure was applied. Another set of filaments was observed inside the core. It has been found that the isolated mixing regions have complex multi-structures consisting of various KAM tori.