Lycosin-I, a peptide toxin derived from spider venom, has been demonstrated to be a promising candidate for the inhibition of tumor cell growth in vitro and in vivo by interacting with and penetrating the cell membrane. Owing to the shortage of an efficient characterization strategy, however, there is still a lack of detailed knowledge about the distribution of the stoichiometric composition information on this peptide in solution and on lipid membrane prior to the cellular uptake process, which is fundamentally important for the understanding of the anticancer mechanism. In this work, with objective-type total internal reflection fluorescence microscopy (TIRF), the distribution of the stoichiometric composition of lycosin-I in different solutions as well as on the lipid membrane was explored extensively on the basis of a statistical single molecule fluorescence intensity analysis for the first time. It was found that lycosin-I is mainly present in a monomer state in diverse physiological solutions regardless of the concentration of the peptide and the incubation time. However, on the lipid membrane, the fraction of small size oligomers increased as a function of time. Fusion of movable peptide molecules to those peptide oligomers with restricted motion on the lipid membrane was also observed.