A leaching kinetics was conducted for the purpose of recovery of praseodymium in sulfuric acid (H2SO4) from REE slag concentrated by the smelting reduction process in an arc furnace as a reactant. The concentration of H2SO4 was fixed at an excess ratio under the condition of slurry density of 1.500 g slag/L, 0.3 mol H2SO4, and the effect of temperatures was investigated under the condition of 30 to 80 oC. As a result, praseodymium oxide (Pr6O11) existing in the slag was completely converted into praseodymium sulfate (Pr2(SO4)3·8H2O) after the leaching of 5 h. On the basis of the shrinking core model with a shape of sphere, the first leaching reaction was determined by chemical reaction mechanism. Generally, the solubility of pure REEs decreases with the increase of leaching temperatures in sulfuric acid, but REE slag was oppositely increased with increasing temperatures. It occurs because the ash layer included in the slag is affected as a resistance against the leaching. By using the Arrhenius expression, the apparent activation energy of the first chemical reaction was determined to be 9.195 kJmol-1. In the second stage, the leaching rate is determined by the ash layer diffusion mechanism. The apparent activation energy of the second ash layer diffusion was determined to be 19.106 kJmol-1. These relative low activation energy values were obtained by the existence of unreacted ash layer in the REE slag.