Exfoliation and assembly of 2-D materials have been extensively investigated to exploit their performances in the area of catalysis, magnetics and electronics etc., but novel strategies are highly desired to shorten their preparing period and to improve their environment-friendliness. Here we introduce a simple and green hydrothermal process to fabricate ultrathin layered beta-Ni(OH)(2)/g-C3N4 nanohybrids for photocatalytic water splitting application. Different from conventionally separated exfoliation and assembly, the exfoliation of g-C3N4, formation of beta-Ni(OH)(2) as well as their self-assembly can be one-pot achieved by simple hydrothermal treatment of nickel dichloride and bulk g-C3N4 free of any organic agents for 12 h. Moreover, the thickness of beta-Ni(OH)(2) nanolayer can be adjusted by controlling the concentration of nickel precursors. The as-obtained nanohybrids have been in detail characterized to have a strong interaction of the assembly components, among which the g-C3N4 is active for photocatalytic water splitting, and beta-Ni(OH)(2) nanolayer is efficient for acceleration of activation of water oxidation. Their photocatalytic water splitting performances are evaluated in the presence of sacrificial agents, based on which the assembly of beta-Ni(OH)(2) nanolayers on the surface of exfoliated g-C3N4 lamella is found to obviously promote the charge separation, water oxidation kinetics as well as final photocatalytic performances compared to the pristine g-C3N4. The direct water splitting is also achieved with the AQY of 1.48% under 405-nm visible light irradition. The one-pot hydrothermal method introduced here may be an alternative strategy for g-C3N4, a promising photocatalytic water splitting material, to construct highly efficient solar energy conversion composite systems based on its exfoliated lamella. (C) 2016 Elsevier B.V. All rights reserved.