Composites materials combining nitrogen-doped carbon (NC) with active species represent a paramount breakthrough as the alternative catalysts to Pt for oxygen reduction reaction (ORR) due to their competitive activity, low cost and excellent stability. In this paper, a simple strategy is presented to construct the graphene oxide-polydopamine (GD) based carbon nanosheets. This approach does not need to modify graphene and use any catalyst for polymerization under ambient conditions, and the obtained carbon nanosheets possess adjustable thickness and uniform mesoporous structure without using any template. The thickness of GD hybrids and carbonization temperature are found to play crucial roles on adjusting the microstructure of the resultant carbon nanosheets and, accordingly their ORR catalytic activity. The optimized carbon nanosheet generated by 5 nm thickness of GD hybrid after 900 oC carbonization exhibits superior ORR activity with an onset potential of -0.07 V and a kinetic current density of 13.7 mA cm-2 at -0.6 V . The unique mesoporous structure, high surface areas, abundant defects and favorable nitrogen species are believed to significantly benefit the ORR catalytic process. Furthermore, it also shows remarkable durability and excellent methanol tolerance outperforming those of commercial Pt/C. In view of physicochemical versatility and structural tunability of polydopamine (PDA) materials, our work would shed a new light on the understanding and further development of PDA-based carbon materials for highly efficient electrocatalysts.