a b s t r a c tRunning impellers have a great influence on the velocity distribution and the flow structure in an oxidation ditch (OD). In this paper, the dimensional analysis and computational fluid dynamics methods were used to study the influence of rotational speed of impellers on the average velocity of the flow in an OD. The time-averaged Navier-Stokes (N-S) equations with a three-dimensional (3D) renormalized group (RNG) k-ε turbulence model were solved by FLUENT6.3.26 for the velocity and pressure fields. By the dimensional analysis and simulation results, the relation between the rotational speed of impellers and the average velocity of the flow was obtained, which can provide a reference in the designing of ODs.