A polyester fiber that can be used for an airbag fabric is disclosed. The polyester fiber has an elongation of 1.65% to 2.5% when tensile strength of 1.0 g/d is applied after heat treatment at 185° C. for 2 minutes, and additionally elongates in the range of elongation from 0.5% to 5% at the range of tensile strength from 8.0 g/d to the maximum strength. The polyester fiber simultaneously has low initial Young's modulus and excellent mechanical properties, and thus, it may provide excellent packing, dimensional stability, and excellent air cut-off effect, and simultaneously, minimize impact applied to a passenger thus safely protecting a passenger.