By using scanning tunneling microscope induced lumines-cence(STML)technique,we investigate systematically the bias-polarity dependent electroluminescence behavior of a single platinum phthalocyanine(PtPc)molecule and the electron excitation mechanisms behind.The molecule is found to emit light at both bias polarities but with dif-ferent emission energies.At negative excitation bias,only the fluorescence at 637 nm is observed,which originates from the LUMO→HOMO transition of the neutral PtPc molecule and exhibits stepwise-like increase in emission in-tensities over three different excitation-voltage regions.Strong fluorescence in region(Ⅰ)is excited by the carrier injection mechanism with holes injected into the HOMO state first;moderate fluorescence in region(Ⅱ)is excited by the inelastic electron scattering mechanism;and weak fluorescence in region(Ⅲ)is associated with an up-conversion process and excited by a combined carrier injection and inelastic electron scattering mechanism involving a spin-triplet relay state.At positive excitation bias,more-than-one emission peaks are observed and the excitation and emission mechanisms become complicated.The sharp molecule-specific emission peak at～911 nm is attributed to the anionic emission of PtPc originated from the LUMO+1→LUMO transition,whose excitation is dominated by a carrier injection mechanism with electrons first injected into the LUMO+1 or higher-lying empty orbitals.