The mannose binding lectin( Crocus sativus lectin, CSL) was subjected to various chemical modifications in order to detect the amino acid residues involved in the hemagglutinating activity of S. Cerevisiae and carbohydrate binding activity. It showed that there was no relation between modification of cysteine and activity of CSL. Modification of arginine, tyrosine and histidine residues led to the decrease of its activities while CD spectra were not changed obviously, indicating that these amino acids residues were active in CSL molecule. However, modification of carboxyl groups led to the decrease of hemagglutinating activity and increase of carbohydrate binding activity notably. CD spectra indicated that glutamic acid and aspartic acid played an important role in CSL steric structure and the effective carbohydrate binding sites increased after the chemical modification of carboxyl groups.