This paper presents the design of a 26-40 GHz monolithic doubly-balanced mixer for high-speed wireless communication. A modified Marchand balun is used to expand the bandwidth. A coupled line of the U section improves the port to port isolation and provides the IF-output port. The mixer was simulated and fabricated in 0.15-μm GaAs PHEMT technology;the measurement results agree well with the simulation results.The mixer achieves low conversion loss of 5.9 to 8.6 dB and high isolation over a 26-40 GHz RF/LO bandwidth and a DC-14 GHz IF bandwidth. This paper presents the design of a 26-40 GHz monolithic doubly-balanced mixer for high-speed wireless communication. A modified Marchand balun is used to expand the bandwidth. A coupled line of the U section improves the port to port isolation and provide the IF-output port. The mixer was simulated and fabricated in 0.15-μm GaAs PHEMT technology; the measurement results agree well with the simulation results. The mixer achieves low conversion loss of 5.9 to 8.6 dB and high isolation over a 26-40 GHz RF / LO bandwidth and a DC-14 GHz IF bandwidth.