In this work, nitrogen-doped carbon nanodots (N-CNDs) with sizes of 2-6 nm were successfully synthesized by hydrothermal treatment of natural biomass (e.g., fresh grass) at 180 degrees C for 10 h. The synthesized carbon nanodots were subsequently immobilized onto functionalized microporous carbon nanospheres (MCNSs) with an average diameter of similar to 100 nm and a surface area of 241 m(2) g(-1) via a simple hydrothermal process to self-assembly form a carbon-based nanocomposite (N-CNDs@MCNSs) owing to the presence of oxygen (O)-containing surface functional groups. As electrocatalyst for oxygen reduction reaction (ORR) application, our experimental results demonstrated that sole N-CNDs could not form stable electrocatalyst film for ORR measurement owing to their high water dispersion property, while the N-CNDs@MCNSs exhibited high electrocatalytic activity with an onset potential of -0.08 V, superior durability and high resistance to methanol cross-over effect, comparable to commercially available Pt/C electrocatalyst. (C) 2015 Elsevier Ltd. All rights reserved.