<p>The diffusion characteristics of U⁶⁺, Th⁴⁺, Eu³⁺, Sr²⁺, Cs⁺, and Cl⁻, which are major nuclides comprising the high-level radioactive waste (HLW) were investigated in altered granite under oxidizing conditions. Since granite acts as final barrier prevening the migration of radionuclides released from HLW into the biosphere, it is essential to investigate the diffusion behavior of radionuclides in granite to ensure the long-term safety of geological disposal systems. The diffusivity of the nuclides were determined experimentally using a through-diffusion method with synthetic groundwater. The diffusivity ​​of nuclides that did not pass through the altered granite matrix during the diffusion test period were calculated using the pore diffusion coefficients and distribution coefficients. U⁶⁺, Sr²⁺, and Cs⁺ diffused through the altered granite matrix as a result of the concentration gradient of nuclide between source solution and diffused solution. Notably, Sr²⁺ and Cs⁺ were predominantly adsorbed onto minerals containing Fe and Ca, respectively. Th⁴⁺ and Eu³⁺ did not diffuse through the altered granite due to their strong adsorption and low concentration gradients. Cl⁻ diffused through the altered granite and eventually reached a steady-state concentration because of surface charge repulsion with the altered granite surface. We comprehensively analyzed the results of this study together with the diffusion characteristics of nuclides reported in previous studies to interpret the diffusion behavior of nuclides in the rock matrix. We expect the findings of the present study to be used in assessing the safety of disposing of high-level radioactive waste through deep geological disposal.</p>