Thermoelectric oxides have been regarded as promising materials for power generation especially at high temperature. Among them, ZnO systems have been reported as useful n‐type thermoelectrics because of high Seebeck coefficients and electrical conductivities at high temperature. In this study, we investigated polycrystalline ZnO systems doped with different donor ions Al3+ and Ga3+. We investigated reasons of different solution limits and secondary phases and microstructures between these two donor‐doped cases. Thermoelectric properties also showed different tendency between Al3+‐ and Ga3+‐doped ZnO systems because of their different structural properties. As a result, a maximum power factor value 9.18 × 10−4 W/mK2 at 1050 K was obtained in Zn0.98Ga0.02O composition. Copyright © 2012 John Wiley & Sons, Ltd.