To achieve good RF power detection performance, the thermocouple length of thermoelectric RF power sensor is studied in this article. The operation principle of the designed RF power sensor is firstly to convert the RF power into thermal power by terminal RF resistors and then convert the thermal power into DC voltage through many thermocouples based on Seebeck effect. Four the thermoelectric RF power sensors with thermocouple lengths of 200, 300, 400 and 500 μm are fabricated by the standard 0.18-μm CMOS technology. The experimental results demonstrate that measured reflection losses are all lower than -20dB in the operational frequency range of 8GHz to 12GHz. This indicates that the designed RF power sensors have good impedance matching characteristics in X-band. As the thermocouple length gradually increases, the measured sensitivities are 1.21 μV/mW, 1.48 μV/mW and 1.59 μV/mW and 1.61 μV/mW at the center frequency of 10 GHz. And the corresponding signal-to-noise ratios (SNR) are approximately 1.116 × 105/W, 1.114 × 105/W, 1.037 × 105/W and 0.933 × 105/W, respectively. The measurement results show that the increasing of the thermocouple length is helpful to improve the sensitivity of the sensor, but the corresponding signal-to-noise ratio will decrease. The results will help improve the future design of the thermoelectric RF power sensors thermocouple-based.