Disclosed are polyester multi-filamentary yarn useful as a reinforcement for tires and a dipped cord formed therefrom. The polyester multi-filamentary yarn comprises at least 90 mol % of polyethylene terephthalate and has an intrinsic viscosity of 0.70˜1.2 and a tenacity of 5.5˜8.5 g/d with an intermediate elongation difference (E1−E0) between intermediate elongations E0 and E1 amounting to 6% or greater. The polyester dipped cord is produced by subjecting at least two strands of polyester multi-filamentary yarn to first twisting and second twisting, the polyester multi-filamentary yarn comprising 90 mol % of polyethylene terephthalate; forming the strands into a fabric; and treating the fabric with blocked isocyanate and resorcinol formaldehyde latex (RFL), wherein the cord satisfies the following characteristics: a) a tenacity of 5.0 g/d or greater, b) a dimensional stability index (E4.5+SR) of less than 7.0%, c) a breaking elongation of 9% or greater, and d) an intermediate elongation difference (E1−E0) of 3% or less. Showing a harmony of high elastic modulus and low shrinkage, the filamentary yarn and dipped cords are superior in dimensional stability and fatigue resistance, so they can be used as reinforcements for rubber composites such as tires.