This paper presents the design, fabrication, and measurement of a MEMS microwave frequency detector for the first time. The structure integrated on GaAs substrate using MMIC process consists of a microwave power divider, a microwave power combiner, two CPW transmission lines, a MEMS capacitor power sensor and a thermopile. The detector has been designed and fabricated in X band successfully. The sensitivity is 2.86 GHz pF-1 from 19 dBm to 23 dBm by the capacitor, and 0.031 GHz ??V-1 from 10 dBm to 20 dBm by the thermopile, respectively.