A polyester multi-filamentary yarn having a novel internal microstructure, polyester tire cord produced from the yarn and a process for production thereof are provided. The polyester yarn has a novel three-phase microstructure consisting of crystalline, amorphous and mesomorphous portions which are changed to a structure having crystalline and amorphous portions during formation of a tire cord. The polyester resin contains at least 90 mol. % polyethylene terephthalate. The resin is melt-spun and solidified by quenching to produce an undrawn yarn having a birefringence of 0.03 to 0.08, which is then drawn at a total draw ratio of 1.4:1 to 2.2:1 and thermally treated and relaxed. The resulting filamentary yarn is dipped in a rubber solution to produce a tire cord which exhibits excellent dimensional stability and fatigue resistance when it is incorporated into the rubber matrix of a tire.