Sophora alopecuroide trypsin inhibitor (SATI) was purified from the seeds of Sophora alopecuroide L. by affinity chromatography on trypsin-sepharose 4B. The inhibitor is homogenous on SDS-PAGE (12%), which is a single polypeptide chain with alanine as its N-terminal. It has the molecular weight of 18 kD and pI at 9.3. Results also show that the inhibitor has a 1:2 molar inhibitory ratio of inhibitor to trypsin. In further study with the method of modification, the residues of argenine and lysine are found to be the only 2 reactive centers on the inhibitor. Feeding trials on cotton aphid indicated that the inhibitor has relatively strong anti-cotton aphid activity with LC(50) (half lethal concentration) of 87.2 mg/L and LD(50) (half lethal dosage) of 6.796 &mgr;g (feeding for 120 h).