A study of electric-pulse-induced crystallization of Ge2Sb2Te5 (GST) was conducted by in situ scanning electron microscopy observation and resistance measurement. A lateral phase-change memory with a top GST channel connected by two separate underlying electrodes was adopted in this study to easily observe the crystallization process. At a low voltage pulse, randomly distributed nuclei were initiated. At the first growth stage, these nuclei grew fast with the pulse amplitude at a rate of around 60 nm/V and then growth rate slowed down to around 14 nm/V when the grain diameter was closed to film thickness. Device resistance during crystallization dropped by around one order of magnitude, which should be due to amorphous to face-centered-cubic transition.