CdO ceramics with randomly distributed micropores were synthesized by the conventional solid-state reaction method via adding polymethyl methacrylate (PMMA) as the void forming agent. The introduction of microsized pores can significantly affect both electrical and thermal transport properties of the CdO matrix, and possible underlying mechanisms are discussed. Due to the dramatically reduced thermal conductivity, a highest ZT value of similar to 0.51 has been achieved in the porous CdO at about 1000K, increased by about 52% as compared to that of the dense sample without PMMA addition. This work demonstrates that introducing microsized pores is a very simple, low-cost, and efficient strategy to improve the high-temperature thermoelectric performance of CdO ceramics.