The effect of chitosan pretreatment on the dyeing of cotton fiber and silk fiber was investigated. However, it has been difficult to evaluate the effect of the chitosan precisely, since the characterization of the molecular weight and effect of the degree of deacetylation were not elucidated for the application. The treatment effect may change diversely since the chitosan solution viscosity differs a lot based on the chitosan molecular weight. In this study, three chitosan specimens, varying in molecular weight, were applied for the fabric pretreatment in order to investigate the effect of chitosan molecular weight. Also, in order to maximize the efficacy of the chitosan, highly deacetylated chitosan specimens, meeting the deacetylation degree of $100\%$, were selected far the application. The air-permeability change according to the chitosan molecular weight change, influence on the mordanting, color change, and wash fastness change were investigated.