Abstract: Postharvest apple fruits（cv. Golden delicious） were used as the material and treated with different concentrations of riboflavin solution, then the control effects on fruits with Penicillium expansum inoculation, the changes of key enzymes activities and metabolites content in phenylpropanoid pathway and reactive oxygen species metabolism were further investigated. The results demonstrated that riboflavin treatment at low concentration significantly inhibited lesion diameter in apple fruits inoculated with P.expansum, especially treated with 1.0 mmol/L riboflavin. Moreover, 1.0 mmol/L riboflavin treatment significantly improved the activities of catalase, ascorbate peroxidase, glutathione reductase, phenylanine ammonia lyase and peroxidase, promoted the accumulation of hydrogen peroxide, total phenolic, flavonoid and lignin in apple fruits. These findings suggested that riboflavin treatment on postharvest apple fruits could enhance their disease resistance against blue mould by activating phenylpropanoid pathway and reactive oxygen species metabolism.