Resistive switching characteristics of Pt/Ti/Al 2 O 3 /Pt memory devices annealed at various temperatures including 400 °C, 500 °C and 600 °C for 1 h under ambient condition were investigated in the study. The Al 2 O 3 thin films annealed at up to 600 °C for 1 h remain amorphous phase based on the X-ray diffraction (XRD) analyses. As increasing annealing temperature, the forming voltage (activating the pristine device) of the memory device decreases in contrast to the rising trend of the turn-on voltage (switching from high to low resistance state). However, the turn-off voltage (switching from low to high resistance state) is almost uninfluenced by thermal annealing. These phenomena might be attributed to the interdiffusion between Ti and Al 2 O 3 , based on the analyzed results of secondary ion mass spectroscopy (SIMS) and high resolution transmission electron microscope (HR-TEM). Moreover, the thermal annealing process eventually creates high conducting paths with a high density of oxygen vacancies between the two electrodes.