Chitosan was used to improve the dyeability of DP-finished cotton. Cotton fabric was treated with a mixture of chitosan, 4,5-dihydroxy-1,3-dimethyl-2-imidazolidinone (DHDMI), and catalyst in a one-step process. To investigate the effect of molecular weight of chitosan on the dyeability of treated fabrics, six chitosan samples of different molecular weights were prepared by depolymerization with sodium nitrite; 185,300, 73,200, 59,000, 21,000, 14,000, and 3,800, respectively. Chitosan improves dye uptake of direct and acid dyes considerably, and the dye uptake increases with the increase of the molecular weight of chitosan. Reactive dye uptake increases slightly in alkaline condition as the molecular weight of chitosan decreases. Higher dye uptake is obtained in acidic condition than in alkaline condition. Chitosan treatment has no discernable effect on the colorfastness to washing, but decreases the colorfastness to wet crocking by about half a point. And chitosan affects other properties of treated fabric; lower wrinkle recovery, stiffer handle, and higher breaking strength as the molecular weight of chitosan increases.