Communications Research Laboratory (CRL) and National Space Development Agency of Japan (NASDA) jointly developed an airborne imaging radar system consisting of X-band and L-band synthetic aperture radar's (SAR). Both the X and L-band radar's are full-polarimetric, and the X-band one has interferometric capability. Mounted on an airplane, Gulfstream II, the radar system have been operated to take microwave images in Japan. CRL and NASDA is responsible for the calibration of the radar for the X-band and the L-band, respectively. This paper presents the method and preliminary results of the calibration of the X-SAR being carried out by CRL.