In order to research the sorption of thorium(IV) and uranium(VI) ions from aqueous solutions, the granular-activated carbon was modified with nitric acid. The modified adsorbent was identified by BET, elemental analysis, SEM and FTIR method. The influences of different parameters such as pH, contact time, temperature and initial concentration were investigated using batch system. Adsorption of Th(IV) and U(VI) onto modified carbon has been evaluated in terms of equilibrium, kinetics and thermodynamics studies. When the obtained results of thorium(IV) and uranium(VI) sorption onto modified adsorbent was applied to kinetic models, it was observed that the experimental results much better fitted the pseudo-second-order kinetic model. Adsorption process was better explained by Langmuir isotherm in comparison with other isotherms. Thermodynamic data were calculated from the sorption isotherms. Results suggested that the adsorption of Th(IV) and U(VI) on modified adsorbent was an endothermic and spontaneous process.