Abstract The effects of quercetin on inhibiting Penicillium expansum infections of ‘Qinmei’ kiwifruit during postharvest storage were evaluated. Severe damage to P. expansum’s mycelial structure was observed by transmission electron microscope at a quercetin concentration of 0.25 mg/mL. With the quercetin treatment, kiwifruit infected with P. expansum had higher chitinase, β-1,3-glucanase, phenylalanine ammonialyase, polyphenol oxidase and peroxidase activities, as well as nonexpressor of pathogenesis-related gene 1, pathogenesis-related gene 1, chitinase and β-1,3-glucanase expression levels, compared with those of kiwifruit only treated with quercetin or inoculated with P. expansum. Thus, the inhibitory effects of quercetin on the P. expansum blue mold may be associated with its toxic properties against fungal pathogens and inhibition of defense responses.