Abstract: To decipher the mechanism of L-glutamate on postharvest diseases of fruit, cherry tomatoes were treated with L-glutamate solutions at the concentration of 100 mg/L for 10 min and then incubated for various time (0, 12, 24, 36 h). After inoculating with Alternaria alternata, disease incidence of tomato was measured. The effect of L-glutamate on pathogenesis-related proteins (PRs) and its coding gene were investigated. Moreover, the growth of Alternaria alternata in the presence of L-glutamate was observed in vitro. The results suggested that disease incidence decreased significantly when the time interval between treatment and inoculation increased to 24 h. L-glutamate showed no effect on the spore germination and the survival of pathogen in vitro. Meanwhile, L-glutamate could increase the activity of β-1, 3-glucanase, chitinase (CHI) and peroxidase (POD), and induce the expression of LePR1, LePR5, LeCHI3, LeCHI9 and LePOD. In conclusion, the activation of pathogenesis-related proteins might be related to resistance mechanism underpinning L-glutamate induced fruit immunity.