Nanocomposite materials in MgO-CeO2 (Ce=0-100 mol%) system were prepared as the support materials for Ru-based ammonia synthesis catalysts, so that the resultant catalytic activity was enhanced by mixing them together compared with that of the MgO or CeO2 component was solely used as a support. Such high catalytic activity was due to two effects: electrodonating property of partially reduced CeO2-delta to Ru metal and strong metal-support interaction derived from the unique catalyst morphology in this study.