We investigated the damage on the Ge2Sb2Te5 line structure by pulsed-dc stressing with various frequencies. The line immediately burnt out due to Joule heating under constant dc stress 2.5 MA/cm(2). However, when pulsed dc 2.5 MA/cm(2) was stressed at the frequency of 5 MHz, failure due to thermal fatigue damage was observed. At higher frequency such as 10 MHz, no noticeable damage was observed, yet the compositional change in constitutive elements by electromigration was detected. The change of damage mechanism by varying of frequency is explained by the difference in thermal cycling extent in response to the pulsed-current operation at various frequencies, which is computed using a finite-difference method.