Provided is a superfine polyester fiber containing at least 98 wt% of a polyethylene terephthalate component, and characterized by fulfilling the following conditions: (1) the reduced viscosity (etasp/c) is at least 0.80 dl/g； (2) the total fineness is 7-120 dtex, and the single fiber fineness is no more than 0.5 dtex； the toughness parameter (X) indicated by formula (1) is at least 2.0, the tensile strength is at least 3.5 cN/dtex, and the tensile elongation is at least 12%. X = (tensile strength [square root of]tensile elongation)/(total fineness single fiber fineness) ... formula (1).本發明提供一種極細聚酯纖維，其特徵在於：其係聚對苯二甲酸乙二酯成分之含有率為98重量%以上者，且滿足下述條件：(1)對比黏度(ηsp/c)為0.80dl/g以上；(2)總纖度為7dtex以上且120dtex以下，且單紗纖度為0.5dtex以下；(3)以如下式(1)所表示之韌性參數X為2.0以上，拉伸強度為3.5cN/dtex以上，且拉伸伸長率為12%以上，X=(拉伸強度×√拉伸伸長率)/(總纖度×單紗纖度)…式(1)。
