The effect of nonstoichiometry on the thermoelectric properties of a Ag2Se alloy prepared by a mechanical alloying and pulse discharge sintering process has been investigated. The x-ray diffraction pattern confirms that the Ag2Se alloy was synthesized by using a mechanical alloying process. The thermal conductivity and the figure-of-merit (Z) value of the Ag2Se alloy was 0.65 W/K m and 0.282×10-3/K, respectively. Both the Z values of the excess Ag and excess Se alloy were much higher than that of Ag2Se alloy, probably the result of the increased power factor. However, the mechanism for the increase is different with the alloy type. In the case of the excess Ag alloy, the carrier concentration was increased by the excess amount of pure Ag atoms and/or clusters, resulting in an increase in electrical conductivity. To the contrary, for the excess Se alloy, the Hall mobility, which has a crucial effect on Z value, was increased because of the low carrier concentration. As a result, the Z value was improved from 0.282×10-3/K (Ag2Se) to 0.912×10-3/K (Ag2.05Se0.95) and 2.016×10-3/K (Ag1.975Se1.025). Thus, the thermoelectric Z value for silver selenide can be dramatically increased by the presence of a small, nonstoichiometric amount of Ag or Se.