The fast structure development process in the early stage of crystallization in commercial polyethylene terephthalate (PET) fibers during carefully controlled postspinning annealing treatments has been followed. In addition to conventional structure characterization techniques, transmission electron microscopy (TEM) has been employed to probe the sequence of structural events. In the earliest stage of structural development, direct observation showed an embryonic microfibrillar structure, the microfibril diameter being ca. 5 nm. Upon modest heat treatment, the microfibrils become axially punctuated with crystallites. As further crystallization occurs, the crystallites become more perfect and lateral alignment of crystallites takes place, particularly in extensively crystallized materials, that is, higher temperature and/or longer time of heat treatment. The lateral alignment results finally in a lamellar microstructure.