Within the paper a 3D numerical simulation of a centrifugal pump, using the finite volume method, was performed. The flow in centrifugal pumps is described using Reynolds averaged Navies-Stokes equations. In order to achieve numerical stability of calculation, optimization of numerical mesh was performed. The simulation was performed in case of unsteady flow for different flow rates, in order to analyze phenomena in the impeller and volute. such as pressure and velocity distribution.. As a result of calculation, fields of pressure and velocities for different operating points are obtained