Cd1-xMgxO ceramics were synthesized by the conventional solid state reaction method and the effects of Mg substituting on the microstructure and thermoelectric properties of CdO ceramics were investigated. XRD results exhibit pure CdO phase for all samples, however, TEM and EDS measurements confirm the existence of MgO nano-sized second phase in the high Mg-concentration Cd1-xMgxO samples. Considerable reduction in grain size with non-uniform distribution is observed in SEM images when the Mg content increases. Introduction of Mg into CdO leads to a reduction in carrier concentration due to the widening band gap of CdO, therefore the electrical conductivity decreases and Seebeck coefficient increases with Mg substitution. The thermal conductivity is significantly suppressed through substituting of Mg, resulting in enhancements in ZT values for all substituted samples. Comparing to the original ZT value of pure CdO, 0.32, Cd0.89Mg0.11O achieves the highest ZT of 0.46 at 1000 K, which is thought to benefit from the low thermal conductivity due to the Mg substitution and MgO nano-inclusions. (C) 2016 Elsevier B.V. All rights reserved.