A planar miniature CMOS dual balun with unequal coupled-line width for millimeter-wave (MMW) transceiver is reported. The dual balun can be applied to a star mixer or a four-way power amplifier for four-way equal power dividing. The dual balun occupies a small chip area of 0.0252 mm2 and achieves S11 smaller than -12 dB for frequencies of 50~110 GHz. For frequencies of 55~65 GHz, the dual balun achieves S21 of -7.6~ -7.9 dB, S31 of -7.9~ -9 dB, S41 of -7.5~ -7.9 dB, S51 of -7.8~ -8.9 dB, magnitude of amplitude imbalance (MAI) of 0.34~1.1 dB and phase difference (PD) of 179.22°~180.78° for ports 2~3, and MAI of 0.3~1 dB and PD of 178.83°~180.48° for ports 4~5. For frequencies of 75~85 GHz, the dual balun achieves S21 of -9.1~ -10.5 dB, S31 of -10.7~ -12.7 dB, S41 of -9.1~ -10.5 dB, S51 of -10.7~ -12.6 dB, MAI of 1.7~2.2 dB and PD of 179.42°~180.59° for ports 2~3, and MAI of 1.6~2.1 dB and PD of 179.93°~180.73° for ports 4~5. The state-of-the-art results of the proposed dual balun indicate that it is suitable for 60 GHz and 77 GHz communication systems.