As a typical liquid-like thermoelectric material, Cu2 Se not only has good electrical transport performance, but also has low lattice thermal conductivity. Nanoengineering is an effective method to achieve high thermoelectric properties for thermoelectrics. However, nanoscale Cu2 Se particles are extremely easy to be oxidized in the process of synthesizing, thus affecting its thermoelectric performance. In order to study the effect of powder reduction on the thermoelectric performance of Cu2 Se, nano-sized Cu2 Se powders were synthesized by hydrothermal method. Hydrogen argon mixed gas ( hydrogen volume fraction of 5% ) and pure argon were used to purge the powders, respectively. Cu2 Se bulk materials with density above 95% were prepared by spark plasma sintering. The results show that the oxygen content of Cu2 Se bulk is significantly reduced after powder reduction, and there is a 41% decrease in thermal conductivity at 800 K, with a 16% increase in ZT value.