Cinnamaldehyde (CIN) is a promising natural preservative and generally recognized as safe for commodities as well as consumers. In this work, the antifungal effects of CIN on Aspergillus flavus were evaluated both in solid and in liquid culture conditions. Our results indicated that CIN effectively inhibited radial growth, spore production, mycelium formation, and aflatoxin B₁ biosynthesis by A. flavus in a dose-dependent manner. At the concentration of 104 mg L⁻¹, CIN exposure was able to completely inhibit fungal growth as well as aflatoxin B₁ production. Furthermore, the inhibitory activities of CIN were closely connected with the treatment period and the tested fungal species. Compared with the control strains, CIN dose dependently changed the morphology and ultrastructure of mycelium in different degree. Especially, the reduction of hydrogen peroxide was considered to follow the destruction of mitochondrial. Meanwhile, CIN significantly cut the levels of lipid peroxidation and reduced glutathione. The activity of total superoxide dismutase was significantly inhibited after CIN treatment at the end of incubation, whereas the activities of catalase and glutathione peroxidase were opposite. These results indicated that the inhibitory effect of CIN could attribute to oxidative stress alleviation possibly induced by modifications of cellular structure as well as redox status.