During the phase−change process of Ge 2 Sb 2 Te 5 in PRAM (Phase−change Random Access Memory), phase−change dynamics are strongly dependent on the quenching speed, i.e. the cooling speed of melted phase−change material. Here we report the relation between quenching speed of programming pulse and phases of Ge 2 Sb 2 Te 5 in phase−change memory in detail. The existence of critical quenching speed is identified, which determines amorphous and crystalline phases in melting followed by quenching operation.