The endurance performance of a novel confined phase change memory cell with metallic nitride liner is investigated using a transmission electron microscope (TEM). Write endurance has been shown to be substantially improved by this new structure . Memory cells that had been cycled up to 109 were cross-sectioned and studied using the TEM. A majority of the electrically observed endurance failure mechanisms were stuck-SET. Physical failure analysis showed the failed cell is always associated with large void formation and material segregation. In-situ TEM analysis was used to study the void formation, accumulation and movement, where the memory cell was simultaneously operated and imaged in the TEM. Also, an interesting self-healing process of the void was recorded during the set/reset operation.