A frequency detector at X-band based on MEMS technology is presented, which is based on detecting the phase shift between the two different CPW transmission lines. The phase shift, which is proportional to the frequency of the signal, results in the variation of the output power. The variation is detected with two kinds of the microwave power sensors, one is the capacitive power sensor and the other is the thermopile one. Therefore, the detector can offer a wide detectable frequency range. This frequency detector is designed and fabricated using GaAs MMIC technology. It is measured at X-band with 10 dBm and 17 dBm. The sensitivity is 0.56 GHz/pF under 17 dBm by the capacitive power sensor, and 6.67 GHz/mV under 10 dBm by the thermopile, respectively.