Heat-treated metal-nitrogen-carbon (M-N-C) materials are emerging as promising non-precious catalysts to replace expensive Pt-based materials for oxygen reduction reaction (ORR) in energy conversion and storage devices. Despite recent progress, their activity and durability are still far from satisfactory. The activity site and particle size are among the most important factors for the ORR activity of M-N-C catalysts. Extensive efforts have been made to reveal the correlation of active site and activity. However, it remains unclear to what extent the particle size will influence the ORR activity of M-N-C materials. Moreover, to the best of our knowledge, controllable synthesis of M-N-C catalysts with high-density activity sites remains elusive. Herein, we develop a straightforward method to produce a mono disperse and size-controlled Co-N-C (Nano-P-ZIF-67) electrocatalyst, and systemically investigate its catalytic mechanisms. Only by optimizing the particle size, Nano-P-ZIF-67 outperforms the commercial 20 wt% Pt/C regarding all evaluating indicators for ORR catalysts in alkaline media including higher catalytic activity, durability, and stronger methanol tolerance. Nano-P-ZIF-67 is assembled into a cell, and the cell shows a power density of 45.5 mW/cm(2), which is the highest value among currently studied cathode catalysts. We expect Nano-P-ZIF-67 to be a highly interesting candidate for the next generation of ORR catalysts. (C) 2017 Elsevier B.V. All rights reserved.