Isothermal melt crystallization in poly(butylene terephthalate) (PBT) homopolymers and glass-fiber-filled composite was studied by time-resolved simultaneous small- and wide-angle X-ray scattering (SWAXS) methods using synchrotron radiation. During primary crystallization, both average long period and lamellar thickness exhibit a significant decrease. During secondary crystallization, the long period and lamellar thickness show a smaller decrease over a longer period of time, which is approximately linear with log time. Similar to other semicrystalline polymers, the morphology of PET formed during isothermal crystallization can be best described by a dual-lamellar stack model. In this model, primary lamellar stacks are formed first comprising thicker crystalline lamellae, whereas secondary lamellar stacks are formed later between the primary lamellar stacks (as observed by the small change of the interlayer amorphous thickness with time). In addition, no isothermal lamellar thickening is observed. From SWAXS measurements, the appearance of the SAXS peak appears to occur prior to WAXD crystal reflection peaks, suggesting that density fluctuations ape probably present before melt crystallization. The lamellar morphology and crystallization rate is found to be a function of molecular weight. The glass filler in the PET composites has no obvious effect on the lamellar structure, but it appears to act as a nucleating agent.