Au nanoparticles (3.8 ± 0.6 nm) are assembled to sub‐micrometer Au particles (186.3 ± 20.4 nm) and covered with adhesive polydopamine (PDA) as embolism beads (198.8 ± 23.2 nm). Radioactive iodine‐125 is labeled to Au@PDA to introduce the function of intra‐irradiation. For the therapeutic effects of Au@PDA‐125I, Au particles sensitize the radiation to MHCC97H hepatoma cells and tumor‐bearing mice. At the cellular level, after being treated with a relatively low‐dose (5 Gy) γ‐ray, Au‐sensitized radiotherapy (RT) leads to an immediate increase of intracellular reactive oxygen species, accompanying with an increase of cell apoptosis. Due to the intra‐irradiation, self‐healing of RT‐leaded DNA double‐strand breakage is suppressed, inducing a further increase of cell apoptosis after RT treatment. Likewise, 3 cycles of sensitized RT leads to a valid control of tumor volume growth, but Au@PDA‐125I has no harm or radioactive residual on or in the radiosensitive organs, including the thyroid, heart, lungs, liver, and spleen. Additionally, photons emitted from 125I and high X‐ray absorption of the Au element makes the beads suitable for single photon emission computed tomography/computed tomography (SPECT/CT) imaging. Therefore, as theranostic embolism beads, Au@PDA‐125I can both enhance the therapeutic effects of external RT, and provide a real‐time SPECT/CT monitoring of therapeutic time window.