Large impellers are commonly used for a wide range of liquid viscosities, and can mix well in a mixing tank thoroughly. However, large impellers are not the solution for all mixing problems. Therefore, to improve the mixing performance of large impellers, a new method should be investigated. In this research, unsteady agitation is subjected to a Maxblend (R) impeller, which is one of the most popular large impellers, and investigation has been experimentally carried out in two scales, namely small and large scale mixing tanks. A decolorization reaction is adopted to measure the mixing time, and a linear equation which correlates the dimensionless mixing time for unsteady agitation is proposed. The experimental results show that mixing time can be reduced in a stirred tank when unsteady stirring approaches are used. However, from the investigation of these two scales of stirred tank in term of their time periodic interval, we understand the difference of mixing performance for both scales.