The 2-D vortex method and the commercially available CFD software are applied to calculate unsteady hydrodynamic forces on a diffuser pump impeller and the pressure fluctuations caused by the interaction between the impeller and the diffuser vanes. Calculated pressure and fluid forces on the impeller are compared with measured ones. The numerical analysis yields fairly accurate predictions of the fluid forces and the pressure fluctuations in diffuser passages and the pipe systems. It has been demonstrated that the fluid forces caused by the interaction between the rotor and stator vanes are small when the number of vanes on impeller and diffuser is identical. In this case, however, the local pressure fluctuations are larger in diffuser passages and the pipe systems.© 2002 ASME