The first fully monolithic X‐band twin‐dipole antenna mixer consisting of a uniplanar twin‐dipole antenna and a GaAs MESFET single‐gate mixer on the same GaAs substrate fabricated by monolithic microwave integrated‐circuit technology (MMIC) is reported. The total chip size is 5×5 mm2. This circuit received an RF signal of 10 GHz, and down‐converted it to an IF signal of 1 GHz with a conversion loss of 22 dB. The experimental results demonstrate that this topology has potential applications for future low‐cost millimeter‐wave receivers for smart munitions seekers and automotive‐collision‐avoidance radars. © 2001 John Wiley & Sons, Inc. Microwave Opt Technol Lett 30: 436–438, 2001.