Chlorination of poly(m-phenyleneisophthalamide) (Nomex) and poly(p-phenyleneterephthalamide) (Kevlar) fabrics by sodium hypochlorite aqueous solution was investigated. Results indicated that stable N-halamine structures could be readily formed in Nomex, which provided potent, durable, and refreshable antibacterial functions against both Gram-negative and Gram-positive bacteria. However, similar chlorination treatment of Kevlar led to serious hydrolysis of the polymer. The treated fabrics were fully characterized by Fourier trasnform infrared, thermal, and X-ray diffraction studies. Possible mechanisms of this effect were discussed.