AbstractThis paper discusses the forming phenomena under a high‐electric field using an Al‐Al2O3‐Au sandwich structure with a 10 60 nm thick Al2O3 film. When an electric field of ∼106 V/cm is applied to the sandwich structure in a vacuum lower than 1 Pa, the transition of the high‐resistance state to the low‐resistance state, so‐called “forming,” is observed. During the forming process, the injection of Au to the Al2O3 film formed the electric leakage path and the local structural change in both the Al electrode (the negative electrode) and the Al2O3 film occurred. The appearance of the negative resistance seemed to be caused by the destruction and reconstruction of the electric path.The sandwich structure with an A12O3 film implanted with A1+, B+, O+, He+, N+ or Ar+ shows the pre‐forming phenomenon observed in a sample treated in a vacuum for the forming process. The negative resistance was observed at the beginning in this sample. The sample also shows negative resistance in air. The sandwich structure with Au*‐doped A12O3 shows both the pre‐forming phenomenon and the negative resistance in both vacuum and air.