Phase-change behaviors of Ge2 Sb2 Te5 (GST) thin films doped with molybdenum (Mo) or nitrogen (N) were investigated by utilizing in situ electrical property measurement, x-ray diffraction (XRD), transmission electron microscopy (TEM), and x-ray photoelectron spectroscopy. It was found that the Mo doping mainly reduces the resistivity level of amorphous GST while the N-doping raises both the resistivity levels of amorphous and crystalline GSTs. XRD and TEM analyses revealed that the element doping stabilizes the amorphous state of GST and suppresses the grain growth in GST films. This resulted in the increase in recrystallization temperature (T m) and activation energy (Ea) of amorphous-to-crystalline phase transition in GST layers, as revealed by the calculation in terms of Kissinger's theory. The results of data fitting into various percolation models and Johnson-Mehl-Avrami analysis indicated the heterogeneous feature of phase transition process in GST layers that the nucleation first occurs at the atmosphere/sample interface and the recrystallization front advances into the interior of sample in a layer-by-layer manner along the direction of surface normal.