In this work, we present a low cost and environmentally benign hydrothermal method using dried grass as the sole starting material without any synthetic chemicals to directly produce high quality nitrogen-doped carbon nanodot/nanosheet aggregates (N-CNAs), achieving a high yield of 25.2%. The fabricated N-CNAs possess an N/C atomic ratio of 3.41%, consist of three typed of doped N at a ratio of 2.6 (pyridinic):1.7 (pyrrolic):1 (graphitic). The experimental results reveal that for oxygen reduction reaction (ORR), the performance of N-CNAs, in terms of electrocatalytic activity, stability and resistance to crossover effects, is better or comparable to the commercial Pt/C electrocatalyst. The theoretical studies further indicate that the doped pyridinic-N plays a key role for N-CNAs' excellent four-electron ORR electrocatalytic activity.