We report the assembly behavior of an antimitotic cell penetrating peptide (CPP) bound with membrane and in solution using experimental and computational techniques. Our study reveals that the antimitotic peptide spontaneously self-assembles and forms a β-sheet-like structure, which is important for cellular entry. Further, we found that this peptide strongly interacts with both liposome membrane and MCF7 membrane. Interestingly, we observed that, during interaction with both lipid membrane and cell membrane, the peptide fluctuates (oscillates) in a similar pattern. The fluctuations in the fluorescence intensity of fluorescein-labeled peptide in the membrane of liposome and cell are attributed to the fluctuation of fluorescein between non-fluorescent neutral form and highly fluorescent dianion form (i.e., prototropic processes). The rate of fluctuation or oscillation is significantly faster at the cell membrane (0.75 s) than that (1.4 s) at the liposome membrane. © 2019 American Chemical Society.