It is important to understand the switching mechanism of phase change material for failure analysis of PRAM device. In this study, the real time observations of phase transition and void formation mechanism of confined GST structure were investigated using in-situ TEM with multi-pulse AC biasing technique. In-situ SET switching behavior between amorphous state and crystalline state with continuous structural change was successfully observed. Volume shrink of GST, due to the phase transition, induced voids at grain boundary of crystalline phase. Excess Joule-heating after crystallization caused coalescence and migration of voids. These results may give us a crucial clue for endurance failure analysis of PRAM.