Kinetic analysis was applied to investigate the alkaline hydrolysis behavior of poly(ethylene terephthalate) (PET) fibers, e.g., two types of commercial fully oriented yarn (FOY), partially oriented yarn (POY), and high‐speed spun (HSS) fibers. The activation energies obtained from the Arrhenius plots using the topochemical hydrolysis reaction rate constants, i.e., the apparent hydrolysis erosion speed for each PET sample, were concentrated within the range of 62.4　-　69.5 kJmol^-1 without respect to their higher‐order structures. Therefore, it has been deduced that the reaction rate constant depends on the frequency factor, especially on the steric factor at constant temperature. As for the as‐spun HSS fibers with higher crystallinity and lower overall molecular orientation, their rate constants could be well described using the newly defined combined parameter, i.e. the apparent lamellar size normalized in terms of birefringence and crystallinity, which implies that the larger crystallite is more effective in the topochemical hydrolysis erosion of PET fibers while crystallinity and birefringence closely related to the cohesiveness of adjacent molecular chains work negatively. However, the combined parameter proposed could not describe the hydrolysis kinetics anymore after the as‐spun HSS fibers were annealed to enhance the packing of amorphous regions.