Two galactose specific lectins, RSAI and RSAII, were purified from sclerotia of Rhizoctonia solani Kühn and their inhibitory effects were assessed on digestive α-amylase of Pieris brassicae Linnaeus. Results revealed molecular weight of 16 and 15.7 kDa and IC50 inhibitory concentration of 0.87 and 1.68 mg/ml for RSAI and RSAII, respectively. In native-PAGE experiment, both RSAs provoked disappearance of A2 band of the enzyme and decreased sharpness of A1 band. Determination of α-amylase kinetic parameters using Lineweaver-Burk analysis revealed that compared to the control, both lectins reduced Vmax while Km was increased only in the presence of RSAII. Therefore, kinetic parameters indicated non-competitive and mixed inhibitions for RSAI and RSAII, respectively. Moreover, RSAI and RSAII showed the highest inhibition within pH and temperature range optimal for enzyme activity in the control group, i.e., at pH values of 9 (RSAI) and 11 (RSAII) and temperature of 30°C. Incubation of RSAs at 50, 60 and 70°C showed high stability of RSAII while RSAI lost inhibitory potential at 70°C. Results of the current study clearly indicate negative in vitro effects of RSAs on digestive α-amylase of P. brassicae.