The paper present the results of numerical simulations performed for a stirred tank equipped with a PMT type impeller, filled up with a Newtonian fluid. The effects of the grid density and mesh quality and also of the simulation mode on the modelling of fluid flow in a stirred tank were studied. The results are compared with literature data obtained from LDA measurements. It was found that denser numerical grids give more detailed information about generated flow field near the impeller blades. Additionally, better compatibility of predicting and experimental results was obtained in the case of the transient mode simulation, what also demonstrates a significant effect of the angular position of the impeller against baffles on the generated velocity field.