Organic-inorganic heterojunctions are formed by combining 3, 4, 9, 10-perylene tetracarboxylic dianhydride (PTCDA) with molybdenum disulfide (MoS2) monolayer or graphene and their fluorescence properties are studied, respectively. In the MoS2/PTCDA system, the fluorescence intensity is greatly enhanced, because the MoS2 surface promotes the epitaxial growth of PTCDA. However, in the graphene/PTCDA system, the fluorescence intensity is obviously reduced due to the transfer transition of photo-induced electrons in the heterojunction.