The morphology, chemical composition, and crystal structure of aluminum anodized films formed in 85°C aqueous ammonium adipate electrolyte at various forming voltages (20-200 V) were investigated. The relationship between the microstructure and the electrochemical behavior of the anodized oxide film was also explored. The results showed that an amorphous oxide film was formed when the forming voltage was 100 V or lower. Both the increase of film thickness per increase in voltage and the relative dielectric constant were independent of the forming voltage. When the forming voltage exceeded 100 V, crystalline γ′-Al2O3 was observed. The crystalline clusters first appeared at the center of the amorphous anodized oxide film and extended outward as the forming voltage increased. Two well-defined layers could be clearly recognized when the forming voltage was 200 V. The presence of crystalline γ′-Al2O3 was beneficial in raising the relative dielectric constant and the dielectric strength of the anodized oxide films. The relative dielectric constant (13.2) of the anodized oxide film formed at 200 V was higher than that of the film formed at a lower voltage. However, the increase of film thickness per increase in voltage was decreased. Microscopic examination revealed that crystallization induced the formation of defects which caused a decrease in the resistivity of the anodized oxide films.