A simple and rapid method for radiolabeling of three types of Ag NPs has been performed using 125I isotope, with high labeling yields, >90% without disturbing the optical properties. All the factors affecting labeling yield were studied. In order to monitor the in-vivo tissue uptake of radiolabeled Ag NPs using γ-rays, Ag-based radioiodo-NPs with a maximum labeling yield were intravenously injected in normal and solid tumor bearing mice. The preliminary biodistribution study revealed that this new radioiodo-NPs have a high affinity to be localized in the tumor site for a long period of time. The reported highly efficient method provides new radiolabeled Ag-based NPs as tumor-specific agents for both diagnostic and therapeutic applications.