In this paper, we present a theoretical investigation on the electronic structures of bulk ternary mixed chalcogenide Bi2Te2S and its corresponding Bi2Te2S (111) surface thin films based on the first principle within the density-functional theory. The spin-orbit coupling (SOC) included self-consistency. We have found that the effect of SOC significantly changes the electronic properties of bulk Bi2Te2S. For the Bi2Te2S (111) surface thinfilms, we have adopted the method proposed by Park K. et al (2010). Our result shows the presence of robustness states on the electronic structure of Bi2Te2S (111) thin films with the presence of a Dirac Point (DP) below the Fermi level EF that is completely covered in the bulk bands. This result is in agreement with the Topological Insulator (TI) nature in the binary Bi2Te3
