A circuit board includes a substrate, two signal lines, two ground lines and two ground vias. The substrate includes a signal layer, a ground layer and an insulation layer. The signal layer is spaced apart from the ground layer. The signal lines are disposed on the signal layer with a first distance between the signal lines. The two signal lines are symmetrical about a reference line. The ground lines are disposed on the signal layer with a second distance between the first signal line and the first ground line. The second ground lines are disposed on the signal layer with one of the ground lines including two line portions having different widths. One of the ground vias is located at a joint between the two line portions. The two ground vias are symmetrical about the reference line.