An air flow guide device for mounting on a heat dispensing fan includes a U-shaped frame and each of the three sides of the frame has a curve outer surface. A plate is connected between two distal ends of the frame and extends inclinedly from a top edge of the two distal ends of the frame, and a slot is defined between the two distal ends of the frame and the plate. Air flow smoothly flows over the curve smooth surfaces and is sucked by the fan. The plate prevents the exhausted air from being sucked by the fan.