Graphitic carbon nitride (g-C3N4) with Pt co-catalyst was synthesized by in-situ photoreduction and used as a visible light photocatalyst. The size of Pt co-catalyst can be controlled form single atoms to nano-clusters by the induced amount of Pt precursor. The results indicated that compared with nano-clusters, the Pt-0.1-CN (with 0.1 wt% Pt loading amount) which is characterized as single-atom Pt exhibits a pronounced photocatalytic hydrogen evolution capability. For Pt-0.1-CN with single-atom Pt as co-catalyst, the H-2 generation is up to 473.82 mu mol mg(pt)(-1) under visible light irradiation (lambda > 420 nm). The enhanced photocatalytic performance is mainly attributed to the synergistic effect of high light adsorption efficiency, effective charge separation, and high dispersed active sites of Pt atoms. The results of this work highlighted that loading g-C3N4 with Pt single atoms will achieve a maximum utilization efficiency of Pt atoms and an improvement photocatalytic performance.