Nitrogen-doped graphene aerogels with hierarchically porous architectures (N-Gs) are fabricated through the co-assembly of graphene oxide and o-phthalonitrile in a solvothermal process and the thermal treatment of the obtained composites at different temperatures. The structural characterizations indicate that both mesopores and macropores exist in the monolithic N-Gs. More importantly, the architectures, porosities and compositions show obvious dependence on the thermal treatment temperature. As the metal-free catalyst for oxygen reduction reaction in basic media, the sample thermally treated at 750 °C shows the best catalytic performances among the three N-Gs, which is owing to its advantages in the surface areas and content of active N species.