A 26-/77-GHz dual-band branch-line coupler fabricated using standard 0.18-μm CMOS technology is presented. The dual-band coupler is with a compact size of 1.0 × 1.0 mm2. The simulated frequency response is to 110 GHz. The measured frequency response shows a good performance with amplitude and phase imbalnce of ± 0.5 dB and ± 5°, respectively, in the low band. The S21 in the high band is about 7.5 dB The measured isolation and the return loss are better than 15 dB within the two passbands. The on-chip coupler will be useful for the integrated design of a 26/77-GHz CMOS single-chip RF transceiver.