An axial flow fan with a plurality of segment blades is described. The axial flow fan has a base, a hub and a plurality of blade units. The hub is mounted on, or pivots on, the base and supports the blade units. Each of the blade units is connected to a periphery of the hub and extends radially outward from the base has a plurality of segment-blades. A segment space between the segment blades reforms a boundary layer of fluid passing over the segment blades and reduces the thickness of the boundary layer on the blade surfaces. As a result, the separation between the blade surfaces and fluid is avoided to maintain a laminar flow of the fluid adjacent to the segment blades.