Though hydrodynamic characteristic is an important parameter for the optimization of reactors, very little research deals with the hydrodynamics of oxidation ditches. This paper focuses on the hydrodynamics of an oxidation ditch from the point of view of both experiments and simulations. The three-dimensional flow field in the oxidation ditch aerated with surface aerators like inverse umbrella is simulated with computational fluid dynamics (CFD). The two-fluid model and the standard k-epsilon model are used for the turbulent solid-liquid two-phase flow. The distributions of liquid velocity and volume fraction of solid phase are obtained. Experiments are performed in a lab-scale oxidation ditch with particle dynamic analyser (PDA). The comparison between simulations and PDA experiments confirms that the results and the methods employed in this paper are reliable. The liquid and solid have similar flow velocity, while the vertical velocity of solid phase is slightly lower than that of liquid. With the increase of aerator speed, the velocity increases and solid phase disperses more evenly. The flow in the oxidation ditch has noticeable three-dimensional features, especially between the circular channel and the straight channel. And solid phase is ready to settle in these regions. All these results form the basis for the optimization and deep research of oxidation ditches. (C) 2009 The Institution of Chemical Engineers. Published by Elsevier B.V All rights reserved.