Abstract: The flow inside a vane-island diffuser has been studied experimentally using direc-tional probe traverses and time-resolving stereoscopic particle image velocimetry.A classical three holes probe was used covering most of the complete section of the diffuser from hub to shroud and from the pressure to the suction side in order to evaluate the static pressure recovery at five mass flow rates.The difference between theoretical and measurement values of pressure recovery factors shows that the great loss of the static pressure comes from the vaned part when the flow rate is higher than the designed value of the diffuser.Previous 2C-2D PIV measurement results were also given to verify that flow separation area exists on the pressure side under the above two conditions.Six schemes of vortex generators were designed to control the boundary-layer separation.Mean flow characteristics of the scheme obtaining the best pump performance was then analyzed.