Acrylic fibers were graft-copolymerized with N-isopropylacrylamide in aqueous media, using a radical initiator, benzoyl peroxide. The grafting reactions were carried out within the $75{sim}85^{circ}C$ temperature range, and the effects of initiator, monomer concentration, and the amount of fiber on the graft yield were also investigated. The maximum graft yield of 88.6% was reached when grafting was carried out at the benzoyl peroxide concentration of $2{ imes}10^{-3}mol/l$, the N-isopropylacrylamide concentration of 0.5 mol/l, for 8 hours at $80^{circ}C$. The grafted fibers were characterized by nuclear magnetic resonance spectroscopy, infrared spectroscopy, scanning electron microscopy, and thermogravimetry. Scanning electron micrographs showed that homogeneous fiber surface changed to shell-like heterogeneous appearance with increase in degree of grafting. Fiber diameter also increased with graft yield. Moreover, moisture regain and water absorptivity of the grafted fiber were highly enhanced by grafting. The results on the response of grafted acrylic fibers to pH and heat are presented.