A differential signal transmitting circuit board includes a substrate, at least two differential conductive elements, and at least one insulating element. The differential conductive elements are disposed in the substrate. The insulating element is disposed in the substrate. The insulating element is close to or contacted to the differential conductive elements. A material of the substrate has a first equivalent dielectric constant. A material of the insulating element has a second equivalent dielectric constant. The first equivalent dielectric constant is different from the second equivalent dielectric constant.