The present invention relates to a polyester fiber that can be applied to a fabric for airbag, particularly, to a polyester fiber having an elongation of 0.4% or less at the stress of 1.0 g/d, an enlongation of 4.0% or less at the stress of 4.0 g/d, and an enlongation of 7.0% or less at the stress of 7.0 g/d, and an initial modulus of 105 to 130 g/d; a method of preparing the same; and a fabric for airbag prepared therefrom. Since the polyester fiber of the present invention decreases stiffness remarkably and secures superior mechanical properties by having a high strength, high elongation, and high shrinkage, it is possible to provide superior packing property, dimensional stability, and air-tight effect, and to protect passengers safely by minimizing the impact applied to the passengers, when it is used for the fabric for airbag.