Since the light emission from a scanning tunneling microscope (STM) was first observed by Gimzewsld et al [1],STM-induced luminescence (STML) of organic molecules has been attracting lot of attentions due to its potential applications in the organic light emitting diode devices.A mechanism for explaining STML of molecules,called as surface plasmon-enhanced molecular fluorescence (SEMF) was proposed recently [2].In this model,the entire optical process is ideally divided into two steps,which is the excitation of local surface plasmons at metal substrate by tunneling electrons,followed by the molecular excitation and fluorescence associated with the highest occupied molecular orbital-lowest unoccupied molecular orbital transitions.In this paper,more experimental results of the substrate morphology effects on STML from molecules arc presented,in order to further understand this mechanism.Here we studied the molecular luminescence (ML) of porphyrin absorbed on Au thin films,which are deposited at the substrate of HOPG.By employing ambient STM,strong morphology and bias dependent ML behaviors ofporphyrin were observed.The Au films presenting with smooth underlying film and atop cluster can generate clear luminescence spectrum of porphyrin molecules,even under lower tip bias.With the increase of Au film thickness,the big surface morphology fluctuation depresses the light emission,manifested with the absence of the characterized peak in the ML spectrrum.Furthermore,the previous studies show that the intensity of surface plasmons in the overlapping area with the absorption spectrum of molecules probably plays an important role in the excitation of molecular fluorescence [3].However,our results prove that this conclusion is not universal.Even the intensity of surface plasmons is quite weak,strong MF still could be obtained.The observed phenomena above are attributed to the influences of surface morphology,and interpreted with SEMF model.