A series-connected fan includes a first fan and a second fan. The first fan includes a first frame, a first base, first static blades and a first impeller. The second fan includes a second frame, a second base, second static blades and a second impeller. A first underframe of the first frame is connected to a second underframe of the second frame. The first static blades are disposed around the first base and connected to the first base and the first underframe. The second static blades are disposed around the second base and connected to the second base and the second underframe. The first impeller includes a first hub and first rotor blades. The second impeller includes a second hub and second rotor blades. The cross-sectional area of the first hub increases along a direction from the top of the first hub to the bottom of the first hub.