The amorphous-like structures and melting behaviors of ultrathin platinum nanowires are studied by EAM potential by using empirical molecular-dynamic simulation and the dependence of nanowire melting temperature pm soze os pntaomed. When the Lindeman criterion is 0.03, we find that the melting temperature for Pt nanowires is well consistent with the result obtained from the potential energy. Through comparing the Lindemann indexes on each shell, the thermal stability is studied. The results indicate that melting of the cylindrical helical structures starts from the interior atoms and that of the bulklike rectangular structure starts from the surface. We also observe the positions of the atoms at different temperatures to obtain the atomic diffusion and mobility.