In this work, the thermoelectric properties of Se-doped Bi2Te3 are examined using first-principles density functional theory and semi-classical Boltzmann transport theory. Placing a single Se atom on the 3a Wyckoff position lowers the unit cell energy by approximately 3.6 eV, compared to the 6c Te position. The electronic structure of Bi2Te3 has minor changes upon Se doping. At carrier concentration of 1019 cm-3, the optimal thermopower, S, is obtained as 207 and 220 μV/K for n-type and p-type doping, respectively. Unlike the thermopower, the power factor, S
σ/τ, is highly anisotropic for the in-plane and cross-plane conduction. At carrier concentrations of 1019 cm-3, the best power factor is predicted to be around 1.05 and 1.4×1011 W/m·s·K2 for n-type and p-type doping, respectively.