Abstract: An online type microwave power sensor structure is presented based on MEMS technology. The theory, design, fabrication and measurement are given. In order to obtain the low losses and wideband response of the power sensor, an impedance match structure and the open-stub CPW lines are used. The fabrication of this power sensor is fully compatible with GaAs MMIC process. The results show that the sensor has the reflection losses of less than -18dB and the insertion losses of less than 0.45dB in the frequency range from 8GHz to 12GHz. The sensitivity of such a power sensor is 12.0μV/mW at 10GHz frequency.