Single atomic layer nanosheet materials show great application potential in many fields due to their enhanced intrinsic properties compared to their counterparts and newly born properties. Herein, g-C3N4 nanosheets with a single atomic layer structure are prepared by a simple chemical exfoliation method. The as-prepared nanosheets show a single atomic thickness of 0.4 nm and a lateral size of micrometers. The structure and photocatalytic properties of the as-prepared single layer g-C3N4 are then studied. Compared with the bulk g-C3N4, single layer g-C3N4 nanosheets show great superiority in photogenerated charge carrier transfer and separation. Accordingly, the photocatalytic H-2 production and pollutant decomposition activities and photocurrent generation of single layer g-C3N4 nanosheets are much higher than those of the bulk g-C3N4, indicating the great application potential of single layer g-C3N4 nanosheets in photocatalysis and photosynthesis.