It has been known that the natural colorants exhibit good dye-uptake toward cotton, silk, and wool fibers, while they do poorly toward synthetic fibers. This study utilizes the chitosan treatment in order to improve the dye-uptake for the natural fibers and to enable the synthetic fibers, whose dye-uptake levels are low, show better affinity toward the natural colorants. Since chitosan has $-NH_2$ group and -OH group in the structure, the dyeability of the fabric will be improved when the fabric is treated with the chitosan. Cotton fabric as one of the natural fiber fabrics and nylon fabric as one of the synthetic fiber fabrics were selected for this study. 1. In case of cotton fabric, the chiosan treatment takes effect for the Caesalpinia sappan and cochineal, resulting in remarkable ${Delta}E$ increase after dyeing. Chitosan helps in developing dark shade by increasing the uptake of the Caesalpinia sappan and cochineal. It does not, however, participate in the developing of the specific color as does a metallic mordant. 2. In case of dyeing cotton fabric with Gardenia jasminoides, the effect of the treatment with mordant and chitosan is not very pronounced. It is thought that the Gardenia jasminoides uptake is accomplished in a direct manner in the cellulose chains without the aid of mordant. 3. Air-permeability is decreased when the non-mordanted and non-chitosan treated cotton fabric is dyed with Caesalpinia sappan, cochineal, and Gardenia jasminoides. 4. In case of nylon fabric, premordanting and chitosan treatments are not highly effective in promoting the dye-uptake.