The present disclosure provides a dual-band transformer structure, which is suitable for at least two frequencies. The dual-band transformer structure includes a metal layer, a first transmission line, a second transmission line, and a third transmission line. The first transmission line and the second transmission line are disposed on the metal layer. A first end of the second transmission line is coupled to a second end of the first transmission line. A second end of the second transmission line is aligned with an edge of the metal layer, and a first end of the third transmission line is coupled to the second end of the second transmission line. The third transmission line extends away from the edge.