Terahertz (THz) SAR opens opportunity for some new applications, e.g., sub-mm localization based on 3D SAR imaging, that cannot be based on the conventional SAR operating at frequency ranges below THz. For 3D THz SAR imaging, the image formation algorithms play an important role. Two 3D SAR image formation algorithms, Range Migration (RM) operating in frequency domain and Backprojection (BP) operating in time domain are considered in this comparative study. The algorithms are first used to form 3D images from simulated THz SAR data. The comparison is then based on image quality measurements such as range and cross-range resolutions, and peak sidelobe ratio (PSLR). © 2021 IEEE.