Abstract: Fabrication of the heterojunction composites photocatalyst has attained much attention for solar energyconversion due to their high optimization of reduction-oxidation potential as a result of effective separation ofphotogenerated electrons-holes pairs. In this review, the background of photocatalysis, mechanism ofphotocatalysis, and the several researches on the heterostructure graphitic carbon nitride (g-C3N4) semiconductorare discussed. The advantages of the heterostructure g-C3N4 over their precursors are also discussed. Theconclusion and future perspectives on this emerging research direction are given. This paper gives a usefulknowledge on the heterostructure g-C3N4 and their photocatalytic mechanisms and applications.