Abstract: The preparation and property of Na 131 I therapeutic capsules are studied. The adsorption of different particle size Na 2HPO 4 with different crystallized water is investigated The results indicate that the more the crystallized water, the less the adsorption; the small the particle size, the more the adsorption. The adsorbent amount of >110 mesh 500 mg anhydrous Na 2HPO 4 in No.0 capsule is 270 μL 0.05 mol/L NaOH solution. In order to prevent the volatility of radioiodide, 1% Vc should be included in adsorbent. The disintegrated time of capsule is 5 min. The radiochemical purity and nuclear purity of Na 131 I therapeutic capsule are over 95% and 99% respectively within 30 days after preparation. At the same time, the radioactivity of 20 capsules is in the range of 96.5%～103.5%. The animal experiments show that there is not extraordinary toxicity with Na 131 I therapeutic capsule, and no difference between Na 131 I therapeutic capsule and Na 131 I solution in rabbits thyroid uptake within 24 h.