In the present research, chitooligosaccharides (COS) with molecular weight 800-3000 Da and 90% of deacetylation were chemically modified by grafting 2-chloroethylamino hydrochloride at C-6 position to synthesize angiotensin I converting enzyme (ACE) inhibitory chitin derivatives based on the properties of ACE inhibitors. The synthetic product was designated as aminoethyl chitooligosaccharide (AE-COS) with molecular weight 800.791-4765 Da. Its IC50 value on ACE was 0.8017 mu g/mL. Furthermore, Lineweaver-Burk plots revealed that the inhibition was competitive via obligatory binding site of ACE. Therefore, these results exhibited that substitution of the hydrogen atom at the C-6 position of pyranose residue by the aminoethyl group promotes ACE inhibitory effects of COS. (c) 2007 Elsevier Ltd. All rights reserved.