A comprehensive study on digestive trypsin was undertaken in the larval midgut of Pieris brassicae L. Results of enzymatic compartmentalization showed a significantly higher activity of crude trypsin in the anterior larval midgut rather than posterior-midgut. Using Diethylaminoethyl cellulose fast flow column chromatography a purified trypsin was obtained by specific activity of 21 U mg-1 protein, recovery of 22%, purification fold of 28-fold and molecular weight of 25 kDa. This purified enzyme showed the highest activity at pH 8 and the corresponding temperature of 40°C. However, the specific inhibitors used including 4-(2-Aminoethyl) benzenesulfonyl fluroride hydrochloride, N-p-Tosyl-L-lysine methyl ester hydrochloride and Soybean Trypsin Inhibitor significantly lowered the activity of the purified enzyme in vitro. Moreover, the activity of trypsin and likewise the nutritional indices were significantly altered in the larval midgut feeding upon the leaves treated by 1 mM concentration of each inhibitor in comparison with control. Determination of enzymatic characteristics of insect trypsins is crucial in paving the path for controlling pests by potential natural compounds via transgenic plants.