In the fabrication of phase change memory devices, high temperature thermal processes are inevitable.We investigate the stability of Ge2Sb2Te5 which is the most common phase change material.Phase change memory devices are processed using different temperatures and examinated by transmission electron microscopy and energy dispersive X-ray.Ge2Sb2Te5 lines shrinkage and elements diffusion during thermal process are discussed.Voids in comers of GST lines come into being after 380 degree anneal for 30 minutes which may cause severe problems in stability of phase change memory devices.After thermal process, elements migrations of Ti and Sb are also discussed.