This paper presents a series of RF MEMS thermoelectric power sensors with different resistor sizes. The comparison of their characteristics are analyzed and measured specifically with different power levels. All the adopted thermopiles have same structure sizes to eliminate the influences of thermopiles. What's the difference is that we employ six couples of TaN resistors of 50 Ω but with different sizes. The proposed RF MEMS thermoelectric power sensors are fabricated using GaAs MMIC technology. Measurement results show that the sensitivities of the fabricated sensors A1-A6 are 81 μV/mW, 70.6 μV/mW, 62.4 μV/mW, 58.1 μV/mW, 51.9 μV/mW and 46.7 μV/mW, respectively. This indicates that as the size increases, the corresponding sensitivity has a decrease. This is mainly because that the temperature field of large size resistor affect the temperature of cold area.