A polyester tire cord having improved dimensional stability and methods of manufacturing the polyester tire cord are provided. The method includes forming a discharge yarn, extruded while a polyester containing 85 mol % or more of ethylene terephthalate units is melted and passes through a nozzle; forming an undrawn yarn by spinning the discharged yarn; manufacturing a yarn by multistep drawing the undrawn yarn; plying the yarn by twisting; and dipping the plied yard in a dipping solution, and then drying, drawing, and heat-setting the plied yarn before dipping the plied yarn in resorcinol-formaldehyde latex and again drying an heat-setting the plied yarn. An intermediate elongation of the formed polyester tire cord after vulcanization for 20 minutes at 170° C. is E2, an intermediate elongation of the polyester tire cord before vulcanization is E1, and a difference between E2 and E1 is ΔE. ΔE divided by E1 is 0.5 or less.