Abstract: The Linum usitatissimum trypsin inhibitor( LUTI) had been isolated from naked flax seeds by acetone fractionation,Tris- HCl buffer extraction and Q- Sepharose~(TM) Fast flow.With the purification steps mentioned above,the overall recovery of enzymatic activity of 6.25%,the specific activity of 55.35 U / mg and the purification fold of9.62 for LUTI from crude extraction was achieved.The LC- ESI- MS showed LUTI belonged to Potato trypsin inhibitor family and the relative molecular weight was 8 ku by SDS- PAGE.The trypsin inhibitory activity of LUTI was stable below 70 ℃,as well as p H2.0~ 6.0. The optimum temperature of LUTI was 40 ℃ and the optimum p H was6.0.LUTI was a non- competitive inhibitor by kinetic assay with an inhibition constant Kiof 9.18 × 10~(-4)mol / L and contained a pair of disulfide bond by DTNB assay,which was related with the stability and activity of LUTI.