A novel planar type broadband balun having a symmetrical structure has been proposed. Considering a lossless and uniform material, balun operation is analyzed and impedance matching condition is formulated. Applying the proposed balun configuration, and using CMOS multi-layer structure, a CMOS on-chip balun has been fabricated. The fabricated balun has achieved ultra-broadband performance. Measured characteristics of the balun are return loss of more than 8dB, insertion loss of 1.5 ∼ 6.3dB from the unbalanced to the balanced ports, and the amplitude difference and phase difference between the balanced ports of -0.2 ∼ +1.5dB and 173.5 ∼ 179deg, respectively, over the frequency range of 13 ∼ 110GHz.