The invention provides a planar balun and a multilayer circuit board. The planar balun formed on the multilayer circuit board comprises a first winding with at least one wire turn, wherein the first winding is formed in a first conductive layer and has a first lead wire and a second lead wire, and the first lead wire and the second lead wire are respectively taken as a first balance end and a second balance end of the balun; a second winding with at least one wire turn, wherein the second winding is formed in a second conductive layer and has a third lead wire and a fourth lead wire, the second conductive layer is separated from the first conductive layer through at least one first insulation layer, the third lead wire is connected with ground potential, and the fourth lead wire is taken as an imbalance end of the balun; and a first balance capacitor which is connected between a selected part near to the center of the first winding and the ground potential.