Poly(ethylene terephthalate) fibers with improved mechanical properties and dimensional stability were spun via controlled threadline dynamics by a liquid isothermal bath (LIB) spinning process : followed by postdrawing and annealing. Control fibers were made by unperturbed spinning and posttreatment similar to a traditional spin-draw process. The two sets of as-spun fibers were spun at take-up speed in the range of 2000-5000 m/min. Fiber properties of the as-spun fibers and posttreated fibers of each process were compared. Two commercial tire cords, i.e., conventional tire cord and low shrinkage tire cord, were also included. Unlike unperturbed spinning, the LIB as-spun fibers show unique structural properties of high amorphous orientation, low crystallinity, high strength, and high initial modulus. Moreover, noncrystalline chains are further extended during posttreatment. The posttreated LIB fibers exhibit mechanical properties with tenacity higher than approximately 9 g/d, initial modulus higher than 120 g/d, and ultimate elongation less than approximately 10%. They also demonstrate superior dimensional stability with thermal shrinkage less than 6% and LASE-5 higher than 5 g/d. The overall properties are not obtainable by either the traditional spin-draw process or any modified process that produces low shrinkage tire cord. Unlike the case fbr unperturbed fibers, the mechanical properties of the posttreated LIB fibers demonstrate a strong/dependency on the birefringence of their respective as-spun fibers. There are at least three significant pieces of evidence that strongly indicate the existence of a third phase, referred to as the taut-tie noncrystalline phase (TTNC), in addition to the traditional two-phase model, i.e., crystalline and random amorphous phases. A unique feature involving a high fraction of taut-tie noncrystalline phase (TTNC %) in the LIB as-spun and the posttreated fibers is also found and which is, in fact, achieved neither by the traditional spin-draw nor the commercial tire cord processes. Further, different from the posttreated unperturbed fibers, the posttreated LIB fibers have an enhanced fraction of taut-tie noncrystalline chains with shorter length, which is believed to be one of the important factors leading to the superior mechanical properties and excellent dimensional stability achieved.