Abstract: In order to measure radar cross sections (RCS) of metal plates, the RCSs were measured with a terahertz time-domain spectroscopy (THz-TDS) system and a 0.1THz continuous-wave measurement system respectively. RCS measurement modules were analyzed theoretically. The results show that measurement errors increase with the increase of measurement frequency and angles for a THz-TDS. For 0.1THz continuous-wave measurement system, errors increase with the increase of angles, and errors increase first and then decrease with the increase of the size of metal plates. The minimum error is -0.244dB. The results show that RCSs of metal plates can be obtained with both systems. Advantages of a THz-TDS are large frequency range and excellent angular resolution. Advantages of a 0.1THz continuous-wave measurement system are simpler structure, faster imaging speed and convenience.