The melting process of the 13-particle clusters bound by the Gupta potential was investigatedby using classic microcanonical molecular dynamics simulation .It is found that the 13-particle cluster show a two-step melting process when the values of t he Gupt a potential parameters p and q are suitable .By using the atomic equivalence indexes σi(t),further studies are made .It is found that the first step corresponds to is omerizations involving only surface atoms and the second to isomerizations that involve all atoms .