the present invention provides a polyester fiber of easy dyeability . the propylene chain of polypropylene terephthalte of easy dyeability is incorporated into the main chain of polyethylene terephthalate , and a ppt / pet copolyester having both ethylene and propylene groups can thus be formed . the ppt / pet copolyester is then spun into a copolyester fiber . the copolyester fiber of the present invention can be successfully dyed with a disperse dye at a temperature below 100 ° c . in the absence of a dye carrier .