Proteases are one of the most important digestive enzymes in the midgut of Hyphantria cunea Drury. Proteases are responsible for protein digestion. In the present study, we evaluated the efficiency of some plant inhibitors on proteases in the gut of the H. cunea. Last instar larvae were collected from mulberry trees. The digestive system of the larvae was used as an enzyme source. The total proteolytic and trypsin activity were assessed by the hemoglobin and BApNA, respectively, as the substrate. The evaluation of the total proteolytic and trypsin activities in various pHs showed the highest relative activity at a pH of 11. Also, the inhibitory effect of inhibitors extracted from Alhagi maurorum Medik., Lathyrus sativus L., Vicia faba L., Prosopis farcta (Banks & Sol.) Eig., and Panicum miliaceum L. on the digestive protease of the fall webworm was measured. Protease inhibitors extracted from A. maurorum, P. farcta and P. miliaceum showed negligible inhibition but L. sativus was able to inhibit 34.72% and 100% of the total activity of proteolytic and trypsin, respectively. Also, the total proteolytic and trypsin activities were inhibited by the inhibitor from V. faba, at 22.27% and 100%, respectively. The zymogram pattern of trypsin with nitro-cellulose membranes showed 2 isoforms in the gut of H. cunea. The inhibitor from L. sativus completely inhibited both isoforms. Gel electrophoresis of proteolitytic activity revealed at least 6 isoforms the inhibitor extracted from L. sativus; completely inhibiting some of them. The inhibitor from L. sativus was purified by ammonium sulfate precipitation and gel-filtration. The molecular mass of the inhibitor was determined as 45 kDa. The highest inhibition of trypsin activity by the inhibitor from L. sativus occurred at a pH of 10. The stability of the inhibitor from L. sativus was evaluated at different pHs and temperatures. The results showed that the inhibitor from L. sativus was stable at a pH of 11.0, and showed 45% inhibition on trypsin activity at a pH of 11. Also, this inhibitor revealed stability up to 50°C.