Radiotherapy is one of the effective treatments for cancers. This study presents a potential internal radiotherapy drug, radioactive calcium-phosphate micron-sized particles (RCPMP). The interaction of calcium-phosphate micron-sized particles (CPMP) with hepatoma cells and the cytotoxicity of RCPMP to hepatoma cells were evaluated. The CPMP having the eutectic composition of the CaO-P2O5 system (CaO-47wt% P2O5) was prepared and subsequently subjected to neutron irradiation in a nuclear reactor to yield RCPMP. The main nuclide in the RCPMP was P-32. RCPMP could be stably retained in a serum and culture medium. These CPMP had a high affinity with HepG2 human hepatoma cells. Cells climbed onto the larger CPMP particles, surrounding and embedding them within several days. The submicro-sized CPMP were endocytosed and retained in HepG2 cells. The dose dependent killing effect was obtained in RCPMP treated HepG2 cells. Cells were killed and detached from the flask with little morphological changes under a high dosage of RCPMP treatment. Meanwhile, treating HepG2 cell cultures with a lower dosage of RCPMP led to morphological changes before cell death. This RCPMP was biocompatible and had high killing effect on hepatoma cells.