The authors have studied the plasmon enhanced fluorescence from porphyrin film on a metal substrate, by scanning tunnel microscopy (STM), which results from the electromagnetic coupling of molecules with plasmon field of metal substrate. A phenomenological electromagnetic calculation is presented on the basis of effective dielectric media for the system including molecules and metal substrate by Maxwell Garnet approximation. The substrate material dependences of STM-induced fluorescence from organic molecules are reported. We can reproduce the experimentally observed spectral shape of STM-induced fluorescence from porphyrin film on Au, and describe the material dependent enhancement which is consistent with the observed enhancement for various materials; indium tin oxide, highly oriented pyrolytic graphite, Pt, Au, and Ag. (C) 2009 American Vacuum Society. [DOI: 10.1116/1.3066702]