Cobalt oxide (Co3O4) nanoparticles decorated on mesoporous carbon nitride (Co3O4/MCN) nanocomposites for photocatalytic hydrogen evolution were investigated in this work. MCN was prepared using 3-amino-1,2,4-triazole, high nitrogen content, as a single molecular carbon and nitrogen precursor and SiO2 nanoparticles as the hard template. Complementary characterization techniques were employed to understand the textural and chemical properties of the nanocomposites. The bare MCN showed high photocatalytic activity under visible light irradiation without using any co-catalyst. The photocatalytic activity of Co3O4/MCN with a Co3O4 mass content of 5 wt % presented two times higher than the bare MCN, which is attributed to the enhanced visible-light harvesting and more efficient charge separation. Mechanistic study shows lower electron-hole recombination rate, higher charge separation efficiency occurs after the formation of p-n type heterojunction. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.