This paper presents the research on sensitivity characteristics in the packaged 8–10 GHz GaAs MMIC-based inline RF MEMS power sensors under different temperature and humidity environments. In order to achieve inline self-detection systems in RF receivers and transmitters under different operation environments, the effects of the temperature between 5 °–75 ° and the relative humidity between 25%–95% on the linearity and the sensitivity of the packaged power sensors are investigated. Measured results show that the packaged inline power sensors under different temperature and relative humidity maintain the good linearity of the output response at 8–10 GHz. Furthermore, experiments demonstrate that the effects of the temperature on the output thermovoltages and the sensitivity are larger at 8–10 GHz, which result in the 33.3% increase in sensitivity at 10 GHz for the temperature from 5 ° to 75 °. While the effects of the relative humidity on that are smaller at 8–10 GHz, which result in only the 7.9% increase in sensitivity at 10 GHz for the relative humidity from 25% to 95%.