This detailed investigation of the internal flow within a centrifugal pump, at its design point, has permitted to study the effect of a vaned diffuser on the flow inside the impeller. From the actual results, we can conclude that the impellerdiffuser interaction is limited to the impeller exit and it does not have any upstream influence on the flow. The frontier between the rotor and the stator seems to be not easily crossed. The mixing process of the flow at the impeller discharge is affected by the presence of the diffuser vanes. The first half of the diffuser pitch is characterized by an early mixing of the flow. Whereas, in the second half, the flow entering the diffuser channel is still presenting its periodicity due to the impeller. This results indicates, therefore, that the diffuser performances may be affected by the complex flow coming out the impeller. Further investigations in the diffuser and at different operating conditions are projected to better understand the rotor-stator interaction.