In natural dyeing, a number of supplementary methods have been practiced since deep shades of the dyed fabrics are not developed satisfactorily. The methods include using the mordants effectively or subjecting the fabrics to reiterated dyeing processes. In this study, we obtained deep shades in the dyeing of fabrics using Rhusjara ica as the dyestuff and applied chitosan to the fabric specimens in order to diversify the colors. Silk fibers and PET(polyethylene terephthalate) fibers were pretreated using chitosan, and subsequently dyed using different types of mordants. As the mordanting agents, Al, Sn, and Fe were employed. Various shades have been resulted in since the interactions of the mordants are different toward the silk fibers and PET fibers. In this study, we investigated the effect of the chitosan treatment along with the change of the mordanting agents on the color change for the silk and PET fibers.