A program for mixing calculation was developed based on the commercial CFD code CFX4.It was used in the numerical study of mixing process of a single Rushton turbine in the stirred tank. Coupled and segregated solutions of momentum and mass equation were adopted. The calculated concentration response curve was consistent with the literature data. Both solutions predicted the same change of mixing time

but the value of mixing time from coupled solution was shorter than that from segregated solution.Coupled solution needed much more computational efforts than segregated solution.The mixing process relied on the flow field used for mixing calculation. The value of mixing time was dependent on the position of detection and feeding.When the tracer was fed near the impeller

the mixing time was the shortest.When it was fed near the bottom of the tank

the mixing time was the longest.