Previous work for the authors proved that the performance of a centrifugal pump with radial vanes impeller could be improved by opening a small slot in impeller vanes, especially when the pump operates at part loads, [1,2]. It is due to transportation of fluid from the vanes positive pressure sides via the opened slots to the vanes negative pressure sides, which reduces the size of the generated vortices in the impeller passages. In the present work, the authors investigate the optimum length and location of the opened slot in backward impeller vanes. The governing equations representing the flow field throughout the impeller passages are solved numerically by using the stream function and the primitive variable approaches. The numerical solution is introduced in a code program using Microsoft Visual C++ 6 language. The flow pattern is represented by streamlines in order to compare the vortex size in case of impeller with and without slots at different slot lengths and at different slot positions along the vane. It is found that the vortex size, at flowrates less than the nominal value, depends mainly upon the length and the position of the slot. The slot length and position are changed gradually until the smallest vortex size is reached. The slot with these dimensions is defined as the optimum slot. The optimum slot is achieved at slot length, Sslot, from 18% to 22% of the blade radial length, Lvane=R2 - RI, and at slot position, Rsioi, from 35% to 60% of the blade radial length, Lvane. In previous work the authors had proved that the pressure fluctuations at pump exit relates to the vortices formation inside the vane passages and their shedding mechanism from the impeller exit. Hence, in order to investigate the obtained results from the present code program, the pressure fluctuations at the centrifugal pump exit are measured and analyzed by power spectrum analysis. The experimental study proved that the pressure fluctuations are affected by the slot location and slot size. The lowest pressure fluctuations have been achieved experimentally when the theoretically determined values of optimum slot location and size are used practically.