The mixing characteristics of complex non-Newtonian fluids in a cylindrical vessel without baffles are determined in this paper. The working fluid has a shear thinning behavior with yield stress modeled by the Hershel-Bulkley model. Effects of the impeller design are explored; it concerns some modifications in the blade of a Rushton turbine mounted centrally at the half of the tank. Different shapes of cuts have been introduced in the blade of impeller, namely: U-, 2U, V- and W-cuts. We focus on the determination of flow patterns, cavern size and power consumption for various shapes and Reynolds numbers. The obtained results revealed that the cut introduced in the blade gives a reduction in power consumption accompanied with a reduction in cavern size than that obtained with the plain blade. The best compromise between energy saving and well-mixed region size allows to select the impeller with V- or W-cut blade as the best one from all cases studied.