Researches of RCS include computational electromagnetics and image-based measurement. Restricted by computer memory and operating time, computational electromagnetic methods are difficult to be widely applied in terahertz band. Inverse Synthetic Aperture Radar (ISAR) imaging technique has been commonly utilized in image-based measurement. The reflectivity images are usually reconstructed by back-projection (BP) algorithm. However, the RCS angular glint is significant in terahertz band, which cannot be revealed in the BP images. Thus, the sub-aperture imaging technique was employed to measure the target RCS in terahertz band in this paper. Numerical simulations were adopted to test the validity of the proposed method.