The inducement and time effects of different concentration of methyl jasmonate(MeJA )on а-amylase inhibitor in tobacco leaves were studied，and the activities of Spodoptera litura (F.)midgut α-amylase was assayed,，which feed on the tobacco leaves after 1-3d MeJA spraying. The results showed that the content of α-amylase inhibitors activity in leaf increased significantly,，after treated with five gradient concentration of MeJA (0.01mmol/L、0.1mmol/L,、1.0mmol/L,、4.0 mmol/L and 10.0mmol/L )；4 m mol/L MeJA induced the strongest response of а-amylase inhibitor activity in tobacco leaves after 48h treatment ,；In vitro assay demonstrated that the Spodoptera litura(F.) midgut а-amylase activity was lower after feed on the treated tobacco leaves.