A derivative of chito-oligosaccharide (COS), N-(2-hydroxyl)propyl-3-trimethyl ammonium chito-oligosaccharide chloride (HTACC), was synthesized using a reaction of glycidyltrimethylammonium chloride (GTMAC) and COS prepared by depolymerization of a fully deacetylated chitosan. COS and HTACC were applied to the cotton fabrics with a pad-dry-cure process using the reaction between the hydroxyl group of cellulose and terminal aldehyde group in COS and HTACC. Their minimum inhibition concentration (MIC) was evaluated, and the antimicrobial activity and durability to laundering of cotton fabrics treated with them were compared. The complete substitution of NH 2 groups in COS with GTMAC was obtained at a 4 : 1 mol ratio of GTMAC to NH 2 in 18 h at 80ยฐC under the presence of acetic acid. MIC values of the 1.04 DS of HTACC and COS were 50 and 400 g/mL, respectively. A cotton fabric treated with 0.2% of HTACC and 1.8% of COS exhibited 100% reduction of bacteria. At the 50th laundering cycle, 0.3% of HTACC and 2.4% of COS indicated 100% bacterial reduction.