Radiometric calibration is considered as an important and inevitable problem in the development and application of low frequency ultra wide band synthetic aperture radar (UWBSAR) system. Due to the characteristics of the UWSAR, the calibration procedure developed for the regular SAR is no longer feasible. With a careful consideration of the calibrating parameters which vary with the change of the frequency and the aspect angle, this paper proposes that the radiometric calibration method is applicable to the low frequency UWBSAR and a model of radiometric calibration is presented. With the help of such a model, the analysis of the system calibration error was conducted.