The manganese oxide (MnO₂) nanocrystals were formed on the surface of graphene oxides (GOs) and multi−walled carbon nanotubes (MWCNTs) through a facial hydrothermal route, respectively. It is found that the similar flower−like MnO₂ nanocrystals covered on both conductive supports. Moreover, more dense and less size of MnO₂ nanocrystals appeared on the surface of MWCNTs, whereas more perfect crystal structures for MnO₂/GOs. Electrochemical measurements showed that both the nanocomposite electrodes exhibited nearly ideal capacitive behavior, and large capacitive value can be obtained for MnO₂/GOs, while high stability for MnO₂/MWCNTs. The high capacitance performance arises from the unique nanostructure of the nanocrystals, which facilitate the contact of the electrolyte and the active materials, and carbon−based materials provide an effective support for the formation of the nanocrystals and conductive pathway for the nanocomposite electrodes.