Abstract: The digital PIV (Particle Image Velocimetry) is a powerful tool for investigation of complex unsteady flow in turbomachinery,but some special image processing techniques should be added to eliminate the influence of thick arcuate access window and very large pollutant particles on it.On the high speed compressor facility,the unsteady flow fields in a high load transonic fan rotor were measured successfully at the speed of 13200 r/min and 16800 r/min,and the instantaneous measurement results clearly show that:it is the tip clearance flow that causes the highly unsteady flow field in rotor tip region;the tip leakage flow is oscillating at all directions,and the interface between the tip leakage flow and the main flow is highly irregular at any instant.The unsteady movement of the tip leakage flow is the essential factor that causes high turbulence intensity in tip region.The tip leakage flow is a kind of high intense vortex flow,which can accelerate the main flow near the vortex core.Besides this effect,highly irregular interface between the tip leakage flow and main flow will also cause some high speed regions around the tip leakage flow.In the 80% corrected speed,these high speed regions become local supersonic,which will cause extra losses.