The present invention relates to a polyester fiber that can be applied to a fabric for an airbag, and particularly, to a polyester fiber having elongation of 0.5% or more at a stress of 1.0 g/d, elongation of 4.3% or more at a stress of 4.0 g/d, and elongation of 7.5% or more at a stress of 7.0 g/d, and an initial modulus of 40 to 100 g/d, a method of preparing the same, and a fabric for an airbag prepared therefrom. Since the polyester fiber of the present invention remarkably decreases stiffness and secures superior mechanical properties, it is possible to provide superior packing properties, dimensional stability, and gas barrier effect, and to protect occupants safely by minimizing the impact applied to the occupants, when it is used for the fabric for an airbag.