Abstract: Vector modulator has been increasingly an important component in millimeter-wave direct carrier modulation and phasedarray radar applications, because of its capability of direct multilevel amplitude and 360^ophase control at millimeter-wave frequencies. In this paper the key parameters on wideband performance are analyzed based on the balanced vector modulator topology. By investigating the effect of the coupling factor of Lange coupler and field-effect tube geometry on wideband symmetry performance, a novel design technique is proposed. Based on the proposed design technique, a wideband design of 40 GHz ~80 GHz vector modulator microwave monolithic integrated circuit is implemented in an commercial 70 nm mHEMT process. The simulation result shows that the phase error is less than 依4^oand amplitude error is better than 依0. 4 dB in the whole 40 GHz range.