Potential-dependent structures of Pt(100)/ionic liquid 1-methyl-3-octylimidazolium hexafluorophosphate(OMIPF_6) interface have been studied by electrochemical scanning tunneling microscopy(ECSTM). The cation OMI+forms ordered structure on Pt(100) surface, which exists in a potential region of about 1.2 V. When the potential is more negative than-0.6 V, it can be seen that the ordered structure transforms to disordered structure. When the potential shifts positively to +0.6 V, the desorption of cations OMI+o...