By applying a backmixing model to the liquid mixing characteristics of vertical vessels with six impellers, the effect of liquid viscosity on the back flow rate and the exchange rate of liquid was experimentally investigated. The vessel diameter D-T was 0.10 m and the impeller diameter D-i was one-half of the vessel diameter D-T. The relationship among the back flow rate f, the feed flow rate q and the exchange rate Q was established as f=Q-q/2 for carboxymethyl cellulose solution and starch syrup solution as an agitated liquid. The modified dimensionless exchange rate of vertical vessels with six impellers Q/(nD(i)(3)A(r)) was proportional to the agitated Reynolds number N-Re to the power of 0.0, 0.50 and 1.0, depending on the range of N-Re.