The present invention provides poly(trimethylene terephthalate) copolymerized with ester-forming sulfonate and a fiber thereof. The polymer of the present invention is high in melting point, small in loss of melting viscosity and has a high molecular weight. The fiber of the present invention is high in toughness, excellent in whiteness and dyeable with a cationic dye, whereby it is useful for clothing, carpets or non-woven sheets, etc.