Zinc sulfide (ZnS) nanoparticles were synthesized from zinc nitrate hexahydrate and thiourea under microwave irradiation. The ZnS-graphene nanocomposites were calcined in an electric furnace at 700 degrees C under an inert argon gas atmosphere for 2 hr. The heated ZnS-graphene nanocomposites were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and UV-vis spectrophotometry. After heat treatment, ZnS-graphene nanocomposites had a more porous and larger surface area, than the unheated ZnS-graphene nanocomposites. The photocatalytic activity of the heated ZnS-graphene nanocomposites in the degradation of organic dyes, such as methylene blue, methyl orange, and rhodamine B, under ultraviolet light at 254 nm by UV- vis spectrophotometer was evaluated and compared with that of the unheated ZnS nanoparticles, heated ZnS nanoparticles, unheated ZnS-graphene nanocomposites. Among the our experimental results as a photocatalyst, the heated ZnS-graphene nanocomposites exhibited remarkably higher photocatalytic degradation of organic dyes as compared to other nanomaterials such as unheated ZnS nanoparticles and heated ZnS-graphene nanocomposites.