The Volume of Fluid method together with the Large Eddy Simulation technique for turbulence flow was used for CFD prediction of liquid flow and free liquid surface motion in a pilot plant cylindrical flat-bottomed stirred tank with an inner diameter of 1m filled with tap water at room temperature. A pitched-blade (45 degrees) down pumping impeller (diameter of 0.33 m) with six blades was used for stirring. The impeller speed was 180 rpm. For simulation of the impeller motion, the sliding mesh method was adopted. The CFD simulations of the flow field predicted a highly complex, multi-dimensional dynamic system exhibiting unstable, pseudo-stationary behaviour. The main advantage of this CFD simulation is the prediction of the velocity flow patterns and the free surface elevation/depression in the whole system at one time. The results for the location and dimensions of the free liquid surface fluctuation correspond well with an experimental investigation by a conductivity method.