The effect of the molecular weight of Vietnamese chitosan on antibacterial activity was investigated by using three chitosans of different molecular weitht [2.600 (water soluble), 50.000 and 187.0000a] but similar degrees of deacetylation about 75 percent. Chitosan was put on 100 percent cotton fabrics by the pad - dry - cure method with three different concentrations [0.1 percent, 0.3 percent and 1.0 percent (o.w.f)]. After antibacterial treatment, fabric samples was evaluated for antibacterial activity by the shaking method in accordance with ASTM E 2149-01 with Escherichia coli (E.coli). Chitosan was verified by IR spectroscopy methods. Effect of molecular weight of chitosan samples on the antibacterial ability of the fabric will be argued through the dependency graph between molecular weight of chitosan and reduction rate of E.coli of fabric at the different concentrations of chitosan.