We tried in this study to numerically analyze the mixing process of high concentration slurry liquids having a plastic rheological property, and in which unevenness of both viscosity and concentration disappear along with agitation despite the initial condition in which the viscosities are different between the upper and lower halves of a stirred vessel. We verified the reliability of the analyzed results by comparing the viscosity and concentration with measurements of mixing time and the power consumption required to attain mixing. We clarified the relation between the mixing time and the initial concentration (viscosity) difference of the two layers or the final concentration of the mixture, both experimentally and analytically. We ascertained that a mixing performance index, N-M', can be well correlated with the Reynolds number, Rem, and the viscosity correction term for each initial concentration difference of the two layers. The numerical analysis enabled us to understand the correlation equations of the mixing process of a heterogeneously viscous system in which the initial viscosity varies through the vessel and to estimate the mixing performance in the vessel from a local point of view.