Currently, it remains a big challenge for developing bifunctional oxygen electrocatalysts with high performance and low cost towards oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Co3O4 as one kind of non-precious metal electrocatalysts has shown promising bifunctional performance in alkaline media, which is greatly influenced by the average particle size. Here, size-controlled synthesis of Co3O4 nanoparticles loaded on N-doped reduced graphene oxide (RGO) was performed by adjusting the Co2+ initial feed concentration, and five Co3O4/N-RGO catalysts with different particle sizes were obtained. Among the prepared samples, the Co3O4/N-RGO catalyst with the average particle size of 12.2 nm displays the best bifunctional oxygen electrocatalytic activities with the Delta E (E-OER,E-10mAcm-2(ORR,-3mAcm-2)) value of 0.75 V, which is the lowest value for the Co-based bifunctional electrocatalysts as reported in the literatures. The catalyst of Co3O4 nanoparticles on N-doped RGO is a promising bifunctional catalyst for fuel cell and water splitting systems. (C) 2017 Elsevier Ltd. All rights reserved.