In this presentation, using first-principles calculation, the function of samarium (Sm) 4f states and Sm-perturbed O 2p states in enhancing the sulfur tolerance of Sm-doped CeO₂ was elucidated. We find that the sulfur tolerance of Sm-doped CeO₂ is closely related to the modification of O 2p states by the strong interaction between Sm 4f and O 2p states. In particular, the availability of unoccupied O 2p states near the Fermi level is responsible for enhancing the sulfur tolerance of Sm-doped CeO₂ compared to the pure CeO₂[1]. To better understand the importance of unoccupied O 2p states, the effect of various 4f shell dopants (Pr, Pm, and Eu) on the activity of surface lattice oxygen are also investigated. This work also hints on the importance of properly engineering the activity of oxygen ion in CeO₂-based materials by adding dopant in order to develop sulfur-tolerant solid oxide fuel cell anode. [1] DH Lim, HS Kim, SP Yoon, J Han, CW Yoon, SH Choi, SW Nam, and HC Ham, Physical Chemistry Chemical Physics 16 (22), 10727-10733, (2014)