A fan includes a fan impeller. The fan impeller includes an inner frame, an outer frame and a disturbing structure. The inner frame rotates along a rotation axis. The outer frame is disposed around the inner frame. The blades are disposed between the inner frame and the outer frame and arranged radially. The disturbing structure has a plurality of protrusion columns disposed on the outer frame, and the protrusion columns are arranged surrounding the blades. The protrusion columns are provided to generate vortexes in the flow field for disturbing the flow as to reduce noise, and the wind pressure can be increased to obtain better operating characteristics.