The prevention of fungal proliferation in postharvest grains is critical for maintaining grain quality and reducing mycotoxin contamination. Fumigation with natural gaseous fungicides is a promising and sustainable approach to protect grains from fungal spoilage. In this study, the antifungal activities of ( E )-2-alkenals (C 5 -C 10 ) on Aspergillus flavus were tested in the vapor phase, and ( E )-2-heptenal showed the highest antifungal activity against A. flavus . ( E )-2-Heptenal completely inhibited A. flavus growth at 0.0125 µL/mL and 0.2 µL/mL in the vapor phase and liquid contact, respectively. ( E )-2-Heptenal can disrupt the plasma membrane integrity of A. flavus via leakage of intracellular electrolytes. Scanning electron microscopy indicated that the mycelial morphology of A. flavus was remarkably affected by ( E )-2-heptenal. Metabolomic analyses indicated that 49 metabolites were significantly differentially expressed in A. flavus mycelia exposed to 0.2 µL/mL ( E )-2-heptenal; these metabolites were mainly involved in galactose metabolism, starch and sucrose metabolism, the phosphotransferase system, and ATP-binding cassette transporters. ATP production was reduced in ( E )-2-heptenal-treated A. flavus , and Janus Green B staining showed reduced cytochrome c oxidase activity. ( E )-2-Heptenal treatment induced oxidative stress in A. flavus mycelia with an accumulation of superoxide anions and hydrogen peroxide and increased activities of superoxide dismutase and catalase. Simulated storage experiments showed that fumigation with 400 µL/L of ( E )-2-heptenal vapor could completely inhibit A. flavus growth in wheat grains with 20% moisture; this demonstrates its potential use in preventing grain spoilage. This study provides valuable insights into understanding the antifungal effects of ( E )-2-heptenal on A. flavus . Key points • (E)-2-Heptenal vapor showed the highest antifungal activity against A. flavus among (C 5 -C 10 ) (E)-2-alkenals . • The antifungal effects of (E)-2-heptenal against A. flavus were determined . • The antifungal actions of (E)-2-heptenal on A. flavus were revealed by metabolomics and biochemical analyses .