Novel g-C3N4/CoO nanocomposite application for photo catalytic H-2 evolution were designed and fabricated for the first time in this work. The structure and morphology of g-C3N4/CoO were investigated by a wide range of characterization methods. The obtained g-C3N4/CoO composites exhibited more-efficient utilization of solar energy than pure g-C3N4 did, resulting in higher photocatalytic activity for H-2 evolution. The optimum photoactivity in H-2 evolution under visible-light irradiation for g-C3N4/CoO composites with a CoO mass content of 0.5 wt % (651.3 mu mol h(-1) g(-1)) was up to 3 times as high as that of pure g-C3N4 (220.16 mu mol h(-1) g(-1)). The remarkably increased photocatalytic performance of g-C3N4/CoO composites was mainly attributed to the synergistic effect of the junction or interface formed between g-C3N4 and CoO.