The effects of unsteady agitation, which was carried out by counter rotation, or raising and lowering of an impeller, on mixing of viscous liquids in an agitated vessel are investigated experimentally. A decoloration reaction is adopted to observe the mixing pattern and to measure the mixing time. By steady agitation, the segregated regions are formed in a vessel equipped with a 4-bladed pitched paddle in the range of relative low Reynolds numbers less than 200. It is demonstrated that these segregated regions are readily destroyed when counter rotation is carried out and are prevented from forming when the impeller is raised and lowered.