The flow structure in the rotor tip region and stall mechanism for a mixed-flow compressor with various tip gaps are investigated in this paper.Steady simulations are carried out for tip clearances of 0.1mm,0.2mm and 0.4mm.Through analyzing the flow fields with various tip clearances,three kinds of flow structures at different tip clearances are summarized.The transverse flow on the blade hub from blade pressure side to suction side exists at the small tip clearance.It then flows towards to the blade tip and meets with the boundary layer flow near the suction side at the trailing edge,generating negative axial velocity region.The tip leakage flow isnt strong enough to cause secondary leakage flow.With the large tip clearance,transverse flow disappears and the radial flow on the blade suction side becomes weaker,but the tip leakage flow is enhanced.The tip leakage flow initiated from the first-half tip chord spills over the leading edge plane of the adjacent blade.At the same time,the leakage flow near the trailing edge rolls into a vortex,leading to serious blockage at the passage exit.Axial velocity contours at the respective near-stall point of the three tip gaps indicate that different stall phenomenon exists when applying small and large tip clearance.The compressor with small tip gap might stall through modal-type stall inception.While,it is inferred that the compressor with the largest tip clearance ramps into stall through spike stall inception.