Abstract: In order to understand the flow details in the tip clearance of a micro axial fan, the aerodynamic performance at the rotation velocity of 5 600 r/min is experimentally measured, and the three dimensional flow fields at 4 flow rates are simulated numerically using a commercial software, the NUMECA. The structure of the tip leakage vortex and the tip separation vortex, the flow rate distribution of the tip leakage vortex, the pressure difference distribution between the two sizes of the blade, and the effect of the tip leakage vortex on the blade tip load are investigated in detail. It is revealed that the tip leakage vortex and the tip separation vortex are identifiable near the tip region, due to the blade tip clearance. As the flow rate increases, the positions where the two vortices occur move downstream. The difference of pressure across the blade tip decreases with the flow rate. From the tip to the outer side wall, the size of the leakage flow speed and the angle of the blade profile (θ) first increase and then decrease. The differential pressure (Δp) on both sides of the blade gap region decreases with the increase of the flow rate, while the leakage flow velocity and rate decrease. Unloading at the blade tip as the major feature of the pressure field enhances when the flow rate increases.