Heteroatom-doped　two-dimensional　(2D)　carbon　materials　have　been　recognized　as　promising　metal-free　catalysts　for　the　oxygen　reduction　reaction　(ORR)　due　to　their　impressive　surface　activity.　Such　materials　are　often　achieved　by　conventional　chemical　vapour　deposition　(CVD)　growth　followed　by　post-treatment　of　heteroatom-doping　or　obtained　by　template-guided　polymerization　and　carbonization　of　organic　substances.　In　this　contribution,　we　report　a　new　strategy　that　allows　a　direct　conversion　of　three-dimensional　(3D)　aggregates　of　guanine　into　naturally　N/O-doped　ultrathin　few-layer　carbon　nanosheets　without　the　use　of　any　template　or　guiding　agent.　Moreover,　by　pre-ionizing　guanine　with　H2SO4/H3PO4,　a　group　of　multiple　heteroatom-doped　(N/O/S,　N/O/P,　and　N/O/S/P)　2D　carbons　can　also　be　readily　realized.　Considering　the　low　cost　of　precursors　and　the　simplicity　of　the　synthetic　procedure,　large-scale　production　of　such　heteroatom-doped　2D　carbons　is　potentially　available.　The　best　catalytic　performance　is　obtained　from　the　N/O/S/P-doped　carbons　which　show　a　half-wave　potential　(E1/2)　of　0.84　V　vs.　RHE　and　a　diffusion-limited　current　density　(jL)　of　5.40　mA　cm−2　in　0.1　M　KOH,　better　than　those　of　most　graphene-based　catalysts　and　even　the　commercial　Pt/C　catalyst.　Systematic　studies　indicate　that　the　observed　superior　ORR　performance　arises　from　a　combined　effect　of　the　multiple　heteroatom-doping,　high　surface　area　and　abundant　structural　defects　of　the　electrocatalyst.　©　The　Royal　Society　of　Chemistry.