Strong acid cation-exchange membranes were obtained by irradiation grafting of acrylic acid (AA) and sodium styrene sulfonate (SSS) onto high-density polyethylene (HDPE). Thermal and chemical stability of the cation-exchange membranes was investigated. The effectiveness of sulfonate-containing films was conformed in inducing high resistance to oxidative degradation. Thermal stability of the grafted HDPE was weaker than HDPE as detected by TGA analyzing technique. Char residue by TGA of the grafted HDPE is greater than that of HDPE. It shows that the branch chains including –SO,3,Na and –COOH was grafted onto the backbone of HDPE, and thus give a catalytical impetus to the charring. Crystallinity of the grafted membranes decreased with increasing grafting yield of the membrane samples. Is is supposed that the decreased crystallinity is due to collective effects of the inherent crystallinity dilution by the amorphous grafted chains and disruption of spherulitic crystallites of the HDPE component.