We studied the leaching kinetics of recovering neodymium in sulfuric acid from the rare earth elements (REE) slag concentrated by smelting reduction from a magnetite ore containing monazite. The leaching kinetics on neodymium was conducted at a reactant concentration of 1.5 g REE slag per L of 0.3 M H2SO4, agitation of 750 rpm and temperature ranging from 30 to 80 ℃. Neodymium oxide included in the REE slag was completely converted into neodymium sulfate phase (Nd2(SO4)3) in H2SO4 after the leaching of 5 h, 80 ℃. As a result, the leaching mechanism was determined in a two-stage model based on the shrinking core model with spherical particles. The first step was determined by chemical reaction, and the second step was determined by ash layer diffusion because the leaching of REEs by the first chemical reaction increases the formation of the ash layer affecting as a resistance against the leaching. By using the Arrhenius expression, the apparent activation energy of the first chemical reaction step was found to be 9 kJmol^(-1). After the first chemical reaction, leaching reaction rate was determined by the ash layer diffusion. The apparent activation energy of ash layer diffusion was found to be 32 kJmol^(-1).