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