A high-power three-antenna measurement system is successfully developed for calibrating the complex antenna factor of low-sensitivity sensors. First, the system is compared with a conventional three-antenna measurement system using double-ridged guide horn antennas, and the validity of the high-power system is confirmed. Then, the low-sensitivity optical electric field sensor is calibrated by this system and this result is compared with the standard field method. A good agreement between these results is obtained. The high-power three-antenna measurement system is therefore found effective for calibrating the complex antenna factor of low-sensitivity sensors.