Topological surface Dirac states (TSDSs) generated in three-dimensional topological insulators (3D-TIs) are currently of significant interest for new science and advanced technologies. In contrast to many other thermoelectric materials, 3D-TIs exhibit a significant potential to achieve a large enhancement in thermoelectric power factor (PF = sigma S-2) due to their special topological symmetry. However, only limited experiments and discussions have been made so far for elucidating the thermoelectric properties of TSDS. Herein, we report a large S and PF observed for high-quality single-crystal flakes of Sn-Bi1.1Sb0.9TeS2 (Sn-BSTS). Accurate interpretations that the energy-dependent relaxation times tau(E) play an important role in thermoelectrical transport of 3D-TIs are provided. Among 3D-TIs, Sn-BSTS has the highest bulk insulation and shows intrinsic TSDS transport without bulk contributions, along with its hallmark of quantum integer Hall effect at high temperatures. Based on the longitudinal/transverse electrical transport and the thermoelectric coefficient, tau(E) proportional to E-0.21 is accurately deduced. As a consequence of the energy-dependent tau(E), a large enhancement in both S and PF is obtained (S = 58 mu V K-1 and PF = 5.0 mW m(-1) K-2 at 77 K), leading to a large increase of 160% for S and 280% for PF when compared to those of graphene at 77 K. The potential thermoelectric performance of the pure TSDS is discussed based on the Boltzmann transport equations.