Nanotechnology may be applied in medicine where the utilization of nanoparticles (≤100 nm) for the delivery and targeting of theranostic agents is at the forefront of projects in cancer nano-science. This study points a novel one step synthesis approach to build up polyethylene glycol capped silver nanoparticles doped with I-131 radionuclide (131I-doped Ag-PEG NPs). The formula was prepared with average hydrodynamic size 21 nm, zeta potential – 25 mV, radiolabeling yield 98 ± 0.76%, and showed good in-vitro stability in saline and mice serum. The in-vitro cytotoxicity study of cold Ag-PEG NPs formula as a drug carrier vehicle showed no cytotoxic effect on normal cells (WI-38 cells) at a concentration below 3 μL/104 cells. The in-vivo biodistribution pattern of 131I-doped Ag-PEG NPs in solid tumor bearing mice showed high radioactivity accumulation in tumor tissues with maximum uptake of 35.43 ± 1.12 and 63.8 ± 1.3% ID/g at 60 and 15 min post intravenous (I.V.) and intratumoral injection (I.T.), respectively. Great potential of T/NT ratios were obtained throughout the experimental time points with maximum ratios 45.23 ± 0.65 and 92.46 ± 1.02 at 60 and 15 min post I.V. and I.T. injection, respectively. Thus, 131I-doped Ag-PEG NPs formulation could be displayed as a great potential tumor nano-sized theranostic probe.