A miniaturized multi-layer balun includes a pair of capacitive elements, at least one section of broadside coupled lines connected in series to an unbalanced and two balanced ports through a pair of transmission lines. Each section has first and second coupled lines. A ground connection is located between two central second coupled lines, and connected to a ground. By means of a multi-layer structure and the addition of a ground connection, the balun of the invention can be fabricated with five conductor layers. This not only greatly decreases the size of the balun device, but also enhances the stability of the device. From the measured return loss and differences in magnitude and phase to the frequency response, it shows that the balun of the invention has good impedance match.