The fourth-order central finite difference scheme was applied to the convection terms in three-dimensional Navier-Stokes equations to simulate directly the flow fields in a medium Reynolds number (Re=4,500) non-baffled stirred tank. The multi-block method was used for the computation. The rotational cylindrical coordinates with a constant angular velocity were used for the flow region including impellers and a disc. The usual cylindrical coordinates were applied to the other flow regions. The calculation was performed tosimulate the phenomena occuring in the case of water in a period of 64 seconds, which is sufficient time for obtaining various flow mechanisms and time averaged flow characteristics in a stirred tank. Visualization of the calculated results shows the mechanism of large scale vortices, their interactions and merger. The calculated results are in good agrement with the measured data. This means that the simulation well describes the fluid motion in a real stirred tank with impellers of complex geometry.