The effect of the rheological property of fluids on mixing time in a stirred vessel was experimentally investigated using by the redox reaction of iodine and sodium thiosulfate. An aqueous solution of carboxymethylcellulose (CMC) was used as a non-Newtonian fluid having a weak shear-thinning rheological property, while an aqueous solution of glycerol was used as a Newtonian fluid. No significant difference in mixing patterns between CMC and glycerol solutions could be seen. When Re or Re(app)>215, a cylindrical circulation zone appeared around the impeller axis, and this zone determined the mixing time. On the other hand, when Re or Re(app)<195, no distinct cylindrical circulation zone was observed and the upper half of the vessel was initially decolorized. Consequently, mixing in the lower half of the vessel determined the mixing time. Non-dimensional mixing time of non-Newtonian fluid was shorter than that of Newtonian fluid under almost the same Reynolds number condition.