Dyeing properties of poly(ethylene terephthalate) and cotton modified with cationic agents were investigated. N-methylol compounds such as N-methylolacrylamide(NMA) are highly reactive toward cotton in the presence of Lewis acid catalysts. Modified cotton with NMA has the pendant double bond as a site for Michael addition to various amines such as ethylene diamine(ED), methylamine and so on. NMA is expected to react with terminal hydroxy group of PET, and ED causes the aminolysis of ester groups of PET main chain besides the modified PET with NMA. Therefore PET and cotton modified with NMA and ED have acid dyeing sites and cotton had a good dyeing property for acid dye even at low concentration of ED. High concentration of ED was needed for acct dyeable PET in exchange for the physical properties of the fiber. PET modified with ED have amino functional groups which act as ligands to transition metals and form metal chelate resin. This enables PET to be dyed with 1:1 metal complex dye.