A millimeter-wave CMOS on-chip stacked Marchand balun is presented in this paper. The balun is fabricated using a top pad metal layer as the single-ended port and is stacked above two metal conductors at the next highest metal layer in order to achieve sufficient coupling to function as the differential ports. Strip metal shields are placed underneath the structure to reduce substrate losses. An amplitude imbalance of 0.5dB is measured with attenuations below 6.5dB at the differential output ports at 30GHz. The corresponding phase imbalance is below 5 degrees. The area occupied is 229µm × 229µm.