Chito-oligosaccharide (COS) was prepared by depolymerizing chitosan using sodium nitrite. The COS has many advantages in the application to fabrics compared with high molecular-weight chitosan, such as soft handle and uniform distribution. to introduce the reactive groups to COS, we prepared acrylamidomethyl chito-oligosaccharide (COS-NMA) by the reaction of COS with N-methylolacrylamide (NMA) in acidic medium, and the reaction product was characterized by $^1$H-NMR analysis. Cotton fabrics were treated with COS-NMA by a two-step process: the application of an acidic COS-NMA solution to cotton fabrics in the first step and then its reaction in an alkaline medium. Antimicrobial activity of the COS-NMA treated cotton fabrics was excellent. Durability to laundering of the treated fabrics was high enough to retain antimicrobial activity after twenty cycles of laundering.