Abstract: Cinnamon essential oil (CEO), a natural plant extract, has been regarded as a potential alternative to chemical fungicides. In this study, the antifungal and antiaflatoxigenic efficacy of the vapor phase of CEO against Aspergillus flavus were determined. The results indicated that the inhibition of CEO vapor on the growth of mycelia and spores depended on the oil vapor concentration. Moreover, the diameters of A. Flavus colony after removing CEO vapor were also evaluated. A reduction in percentage inhibition was observed after removing the antimicrobial atmosphere and the inhibition effect was found to be static for A. flavus. Besides, the antifungal efficacy of the CEO vapor was strongly affected by water activity (Aw). CEO vapor inhibited the growth of the mycelia and spores when Aw was ≤ 0.96. During antiaflatoxin investigation, a gradual decrease in mycelia dry weight and AFB₁ production by A. flavus in the liquid culture was found on increasing the concentration of CEO in the vapor phase. The CEO vapor was 131.74 μL/L exhibited a complete inhibition on the mycelial growth and AFB₁ synthesis. The essential oil vapor was also investigated to determine morphological changes of A. flavus. The results showed that the number of the spores was obviously decreased and mycelia of A. flavus brought shrinking and stained readily with cotton blue lactophenol solution. This study shows the potential application of the vapor phase of CEO in food industry.