We investigated the isothermal crystallization of poly(ethylene terephthalate) by time-resolved small-angle X-ray scattering (SAXS) using synchrotron radiation. At the early stage, the SAXS intensity decreased monotonously with scattering angle (stage I). Then the SAXS profile showed a peak and the peak position progressively moved to a wider angle with crystallization time (stage II). Finally, the peak intensity rapidly increased, keeping the peak angle constant (stage III). The Guinier and the correlation function analyses suggested that crystalline domains of several tens of nanometers in diameter appear and they consist of the highly disordered lamellae (stage I). In stage II, the crystalline domains grow up to the larger domains in which stacking of lamellae is achieved by insertion of newly formed lamellae between the preformed lamellae. Then they develop to yield the spherulites by attaining a higher ordering of the lamellar stacks in stage III.