Abstract: The flow field of a micro-axial fan stage, which is widely used in cooling systems of electronic devices, is numerically studied. The discrepancy of loss among four operating points which locates in the right of the hump curve is analyzed. It is revealed that in the rotor passage, loss occurs mainly in the region (region I) where tip leakage vortex (TLV) passes through, in boundary area of pressure surface (region II) and in boundary area of suction surface (region III), respectively. With the increase of the flowrate, the region affected by tip clearance vortices gets smaller, while the magnitude of vorticity becomes bigger. Accordingly, the range of high loss region here gets smaller and the magnitude of high loss center becomes bigger. In region II, both the range and the magnitude of high vorticity area get bigger, so the range and the magnitude of the high loss area become bigger. In region III, no obvious change is observed. Overall, loss rises with the increase of the flowrate in the rotor passage. In the stator passage, no significant difference is observed under different flowrates, so in the stage environment total pressure coefficient decreases with the increase of the flowrate.