Dissolution of the irradiated UO₂ target is the first step in process for separation and purification of fission ⁹⁹Mo, the only parent nuclide of ^99mTc and extremely useful tool for medical diagnosis. In this study, the effects of concentration of sulfuric acid (0.5∼2N) and nitric acid (0.01∼0.1N), temperature on UO₂ pellets dissolution rate were investigated. The results of experiments showed that dissolution rate proportioned to the power 2.597 of H₂SO₄ concentration and 1.241 of HNO₃ concentration, and activation energy was evaluated to be 105.2kJ/mol. And also, the optimum operating condition for UO₂ target dissolution was suggested.