The phenomena of vortex formation in relative flow field of a radial flow centrifugal pump, running at zero and part loads of the pump discharge, has been investigated. The flow field is described by two dimensional Poisson's equation, which has been solved numerically, in difference form, by relaxation technique. A physical model for the vortex formation and
shedding mechanisms, at part loads, has been introduced. The effect of vortex shedding, from the impeller, on the pressure fluctuation at the pump exit has been also discussed. Experimental investigation has shown a satisfactory agraement between the measured and the theoretically predicted pressure fluctuations at part flow rates. At high flow rates, the viscosity effect on the vortex simulation model, has to be considered.