Mg2X (X = Si, Sn) thermoelectric materials are promising materials for converting solar-thermal energy to electricity. To investigate the effects of non-stoichiometric compositions and Ag-doping on the thermoelectric properties of Mg2Sn, Mg2Sn thermoelectric materials were synthesized via vacuum melting and the spark plasma sintering (SPS) process. For all compositions with a ratio of Mg:Sn atoms between 67:33 and 71:29, we obtained nearly single-phase Mg2Sn with traces of a second phase. All samples showed the transition of a p- to n-type Seebeck coefficient; however, there were slight differences. To improve the p-type thermoelectric property, Ag atoms were doped into the Mg2Sn system. In the Sn-excess case (Mg:Sn = 67:33), the Seebeck coefficient maintained its p-type characteristic in the entire measured temperature range and the electrical conductivity increased due to the Ag-doping. As a result, the p-type power factor values for Ag-doped specimens increased by as much as 10 times compared to the value for the undoped case. This result could be important for developing p-type thermoelectric polycrystalline in the Mg2X(X = Si, Sn) system. (C) 2011 Elsevier Ltd. All rights reserved.