Abstract: Effect of postharvest citric acid dipping on black spot, reactive oxygen species and phenylpropanoid pathway in muskmelon fruit was investigated.Muskmelon (Cucumis melo L.cv.Yindi) fruit were dipped at different concentrations of citric acid and inoculated with Alternaria alternata.The inhibition effect of citric acid dipping on the lesion diameter, the key enzyme activities and phenylpropanoids accumulation of phenylpropanoid pathway in muskmelon fruits were investigated.The results indicated that citric acid at 2% significantly reduced the lesion diameter of fruit inoculated with A.alternata.Citric acid treatment increased the activities of peroxidase (POD) and phenylalanine ammonia-lyase (PAL) .Citric acid also increased the content of total phenolic compounds, flavonoids and lignin in the fruit.The results also suggested that postharvest citric acid decreased postharvest diseases of muskmelon fruit by accelerating phenylpropanoid pathway.