A new multistage impeller developed For mixing highly viscous fluids (0.1-10.0 Pa.s) in a stirred tank provides several advantages: excellent axial circulation flow, hardly any stagnant zones, and low power consumption. The impeller is composed of a vertical rotating shaft to which are attached, from top to bottom, a pair of two-bladed gate-impellers and one two-bladed paddle impeller. Four baffles are employed to accelerate the axial circulation now in the tank, fn this paper, the geometrical configuration of the multistage impeller is described, and its mixing characteristics for fluids of high viscosity-i.e. the power consumption, flow pattern, mixing time, circulation time, and mixing efficiency-are clarified. For comparison, the same characteristics were also examined in the stirred tanks using anchor, double-helical ribbon, and single-gate impellers. The results showed the multistage impeller to be superior to the other types in that the time required For complete mixing is relatively short and almost no stagnant zones are created. The relationship between the Reynolds and Nusselt numbers for the multistage impeller, as well as for single-gate and four-pitched paddle impellers, was clarified from the laminar to turbulent regions.