Abstract: The orientation of melittin binding to liposomes with of without transmembrand potential was investigated.With The combination of high performance liquid chromatography and mass spectromeny, we detected The membrane association state of melittin by analyzing its proteolytic products .We found that The enzyme could access ail The proteolyticc sites on protein binding to liposome without membrane potential.while one proteolytic site on The N-terminal of The Protein was blocked de it binding to liposome with negative transmembrane potential The results show that melittin reorients in The latter case.