The dyeability. of methacrylic acid grafted poly(ethylene terephthalate) fibers with methylene blue was investigated. The increase in graft yield caused an increase in the dye uptake values of the fiber. Such behavior has been attributed to the carboxyl groups imparted to poly(ethylene terephthalate) fibers by the grafted poly(methacrylic acid) chains. The dependence of the rate of dyeing on the percentage of graft yield was also evaluated with a simple equation. Optical microscope examinations of the cross-section of dyed fibers showed that the grafting proceeded up to the fiber center at or over graft, yields of 27.9 %.