Three polyesters-poly(ethylene terephthalate), poly(2-methyl-1,3-propylene terephthalate-co-ethylene terephthalate), and poly(1,4-cyclohexylene terephthalate-co-ethylene terephthalate)-were preirradiated with Co-60-gamma-rays. Then, acrylic acid and N-vinylformamide were grafted to these Irradiated fibers. Fibers grafted with N-vinylformamide were further hydrolyzed with acid so that the amide groups would convert into amino groups, and they were treated with glutaraldehyde so that aldehyde groups would be introduced. Chitosan or chitooligosaccharide was then grafted to these fibers via either esterification or imine formation. Four pathogenic bacteria-methicillin-resistant Staphylococcus aureus-1 (MRSA), Staphylococcus aureus-2, Escherichid coli, and Pseudomonas aeruginosa-were tested to determine the antibacterial activities of chitosan-grafted and chitooligosaccharide-grafted fibers. The results showed that grafting chitosan via imine formation could achieve a higher surface density for amino groups and give higher antibacterial activity to those four bacteria tested. The antibacterial activity for E. coli was the highest and that for MRSA was the lowest among the four bacteria tested.