Abstract: A dual-band planar monopole antenna mainly includes a microwave substrate, a radiating metallic element, a feeding point, a microstrip line, and a ground plane. The microwave substrate includes a first surface and a second surface. The radiating metallic element is printed on the first surface and has a U-shaped slot thereon. The feeding point is disposed on the radiating metallic element. The microstrip line is connected to the feeding point for signal transmission. The ground plane is printed on the second surface functioning as a ground. The opening of the U-shaped slot is facing the feeding point and separates the radiating metallic element into a first sub-metallic element and a second sub-metallic element for generating a lower operating frequency and a higher operating frequency.