A printed circuit board includes a first printed circuit substrate and a second printed circuit substrate. The first printed circuit substrate includes a substrate layer and a first conductive circuit layer. The first conductive circuit layer is formed on the substrate layer. The substrate layer includes at least two first grooves. The first conductive circuit layer includes at least one signal wire. The first grooves are defined in both sides of the signal wire. The second printed circuit substrate is formed on the first printed circuit substrate. The second circuit substrate includes a third copper layer. A second groove is defined in the third copper layer. The first grooves are opposite to the second groove. The first grooves and the second groove form a space.