In applying a backmixing model of liquid mixing characteristics in vertical vessels with three multiple impellers, the effects of the opening ratio of a perforated partition plate and the stirring speed on the backflow rate and the exchange rate of liquid were experimentally investigated. The vessel diameter D-T was 0.10m and the impeller diameter D-i was one-half of the vessel diameter D-T. In the vertical stirred vessels with three multiple impellers, the relationship of the backflow rate f to the feed flow rate q and the exchange rate Q was established as f=Q-q/2. The exchange rate Q increased in proportion to the stirring speed of impellers n and the opening ratio of a perforated partition plate A(r). The value of the modified dimensionless exchange rate in the vertical vessels with three impellers Q/(nD(i)(3)A(r)) was proportional to the agitated Reynolds number N-Re to the power of 0.20 when the range of the agitated Reynolds number N-R(e) was 1 X 10(3) to 6 X 10(3), and it was almost constant at 0.17 when the agitated Reynolds number N-R(e) was greater than 6 X 10(3).