Provided is a superfine polyester fiber containingat least 98 wt% of a polyethylene terephthalatecomponent, and characterized by fulfilling the followingconditions: (1) the reduced viscosity (.eta.sp/c) is at least0.80 dl/g； (2) the total fineness is 7-120 dtex, and thesingle filament fineness is no more than 0.5 dtex； and(3) the toughness parameter (X) indicated by formula (1)is at least 2.0, the tensile strength is at least 3.5cN/dtex and the tensile elongation is at least 12%.X - (tensile strength × .sqroot.tensile elongation)/(totalfineness × single filament fineness)... formula (1).
