We theoretically investigated the structural and thermoelectric properties of Mg2Si with Al and Sb (Na and B) as n-type (p-type) impurities. Supercell calculations involving relaxation of the atomic positions using an ab initio pseudo-potential method were performed. The formation energies, Eform,i, for the i = Mg, Si, and 4b sites, and consequently, the energetically preferred sites occupied by the impurities, were discussed. The calculated Eform,i were used to estimate the impurity-site occupancy probabilities, pi(T), based on the canonical distribution in the equilibrium state, i.e., pi(T) ∝ exp(−Eform,i/kBT) (Boltzmann constant: kB, temperature: T), and the resultant effects on the carrier concentration. Next, an all-electron full-potential linearized augmented-plane-wave calculation was performed based on the optimized structures, and the temperature dependence of the thermoelectromotive force (the Seebeck coefficient) was evaluated using the Boltzmann transport equation. The calculated and experimental results for n-type doped systems were compared.