A polyester filamentary yarn having a novel internal microstructure, a polyester tire cord produced from the yarn and a process for production thereof are provided. The polyester yarn of the present invention comprising at least 90 mol % polyethylene terephthalate and having a fineness of 3 to 5 denier per filament, wherein said yarn satisfies the following i).about.iii) characteristics, after it has been treated for 3 minutes at temperature of 240.degree.0 C., tension of 0.1 g/d, said yarn satisfies the following iv).about.vii) variations of mircrostructural physical properties: i) a density of 1.38.about.1.39 g/cm.sup.3, ii) a birefringence index in amorphous portions of 0.06.about.0.09, iii) tan .delta. peak temperature of 140 .degree. C. and below, iv) the increment of a percent crystallinity (.DELTA.X.sub.c) of 10.about.20 wt. %, v) the decrement of an amorphous orientation coefficient (.DELTA.F.sub.a) of at least 0.05, vi) the increment of a long period value (.DELTA.LP) of at least 10.ANG., and vii) the decrement to tan .delta. peak temperature (.DELTA. tan .delta..sub.peak) of at least 5 .degree. C. The filamentary yam is dipped in a rubber solution to produce a tire cord which exhibits excellent dimensional stability and fatigue resistance when it has been incorporated in a rubber matrix of a tire.