The efficiency of centrifugal pumps drops sharply when the flowrate is reduced below a threshold value. This is due to a profound change in the flow structure, characterized by a large of portion flow separation near the impeller blades and the formation of energy-intensive recirculation zones. So far, it is not clear how such flow separation may initiate and develop. This study combines state-of-the-art experiments and numerical simulations to explore the onset of flow separation in centrifugal impellers. In particular, a high-frequency particle image velocimetry (PIV) system is used to visualize the velocity field in impeller channels. The continuous relative velocity value and deviation angle relative to the blade surface are displayed before the stall inception conditions. Meanwhile, the validated numerical simulation method is used to compute the flow at similar experimental conditions. The results clearly show a cylindrical vortex band exists near the impeller shroud. As the flowrate decreases, the vortex grows gradually stronger, while moving to the junction between the impeller shroud and blade suction side, and then toward impeller hub along the blade suction side. This growing and moving vortex is the main cause of the flow separation near blade suction side observed in our experiments. Interestingly, the impeller head remains insensitive to this vortex until it causes the flowrate in the adjacent impeller channels to be redistributed. This led us to believe that stalled flow can be detected before it affects the hydrodynamic performances.