In this study, the effect of surface modification of wool fiber with oxygen plasma and chitosan (PC) on the color strength of the samples dyed with cochineal and safflower natural dyes has been studied. According to colorimetric measurements, plasma and chitosan treatments resulted in notable improvements in dyeability of wool fibers. The plasma and chitosan-treated wool sample could be dyed in less time and at lower temperature compared with untreated one. The dyebath pH had a prominent effect on color strength, which showed that the electrostatic force has an important role in dye adsorption. The optimum pH for maximum dye absorption was 3.6. The fastness properties, tensile strength, and elongation at break of dyed wool were improved after PC treatment. Scanning electron microscopy (SEM) images and ATR-FTIR analysis proved the coating of wool scales with a uniform thin layer of chitosan, which caused the improved dyeability of wool fibers with safflower and cochineal.