We observe reversible phase transition phenomena in proto-type chalcogenide random access memory (C-RAM)devices adopting ultra-thin (12nm) Ge2Sb3 Te5 thin film. In this kind of proto-type device, the ultra-thin amorphous Ge2Sb2 Te5 thin film undergoes a crystallization process when a voltage is applied. The polycrystalline Ge2Sb3 Te5 remain unchanged when the voltage is below 0.6 V. A higher power is needed if the transition from polycrystalline to amorphous is expected. The re-amorphization process can be realized by applying a voltage higher than 0.7 V. The threshold voltage Vth and threshold electric field Eth of the transition from the polycrystalline state to the amorphous state in this proto-type device are ～0.7 V and ～ 5 × 105 V/cm, respectively. The programming voltage is significantly reduced compared to the values of C-RAM devices adopting a 200-nm-thick Ge2Sb2 Te5 inset.