Abstract: An excellent Aβ imaging agent must have high affinity for Aβ plaques. The in vitro competition binding assay is an effective method for determining the affinity of unlabelled imaging agents for Aβ plaques. ［¹²⁵I］TZDM was often used as a radioactive ligand in the assay. In this paper, Bu₃Sn-TZDM, a precursor compound of ［¹²⁵I］TZDM, was synthesized by four steps from 4-bromo-aniline. After the precursor compound was labeled with ¹²⁵I, crude ［¹²⁵I］TZDM was obtained. The labelling yield is 62.5%. ［¹²⁵I］TZDM is separated with HPLC and has more than 97% radiochemical purity. The actual yield is 25%.