Herein, novel mesoporous carbon-doped g-C3N4 ultrathin nanosheets (C/CNNS) have been synthesized for the first time through a facile one-step thermal condensation method using agar-melamine gel (AMG) as precursor. A series of characterizations were carried out to explore the structure, morphology and optoelectronic properties of the C/CNNS photocatalyst. The resultant C/CNNS-0.5 exhibited the optimum photocatalytic performance with respect to bulk g-C3N4 by using Rhodamine B, Phenol, Bisphenol A and Phenanthrene as target pollutants under visible light irradiation. Such remarkable enhancement of photocatalytic activity was mainly attributed to the synergistic effect of onion-like carbon (OLC) and ultrathin 2D nanosheets structure. The introduction of OLC could effectively expand visible-light absorption regions. Besides, OLC can act as an electron receiver to facilitate charge separation and inhibit the recombination of photogenerated carriers. 2D nanosheets structure provides more active sites for photo catalytic reactions, which further improve photocatalytic activity of C/CNNS-0.5 photocatalyst. The photocatalytic mechanism of C/CNNS for removing organic pollutants was explored by electron spin resonance (ESR) technique. Much different from the bulk g-C3N4, superoxide radical (O-center dot(2)-) and hydroxyl radical ((OH)-O-center dot) were the two main radicals, while for the bulk g-C3N4, there is only the O-center dot(2)- radical worked in the photocatalytic reaction. (C) 2018 Elsevier Inc. All rights reserved.