A novel broadband circuit model for millimeter-wave Marchand balun using three-conductor coupled line was proposed. Based on the c-mode and π-mode theory, the three-conductor coupled line was analyzed and modeled by the transmission line and several ideal transformers. The model parameters are extracted from the electromagnetic simulation and then the coupled line model is utilized to build the model of the Marchand balun. Using the proposed model, a millimeter-wave Marchand balun was designed and fabricated in GaAs process. The balun works at the frequency range from 15 GHz to 55 GHz, the measurement results show that the balun has good amplitude and phase imbalance performance and verify the model as well.