The binary blends of poly(ethylene terephthalate) (PET) and poly(ether imide) (PEI) were miscible in the melt but showed simultaneous crystallization and liquid-liquid demixing below the melting point. In this study, the morphological structure induced by the coupling of crystallization and liquid-Liquid phase separation in PET/PEI blends was probed by small-angle X-ray scattering(SAXS). For the semicrystalline blends, a drastic intensity rise at low scattering angle superposed the scattering peak associated with the lamellar stacks. This low-angular scattering was attributed to the electron density contrast between the "lamellar stack domains" (LS domains) consisting of alternating crystalline/ amorphous layers and the PEI-rich domains located outside the LS domains. The scattering profiles were deconvoluted by applying the Debye-Bueche model to calculate the scattering intensity associated with the contrast between LS domains and PEI-rich domains. The morphological parameters, such as crystal thickness, amorphous layer thickness, and long period, were subsequently determined from the intensity profile obtained by subtracting the Debye-Bueche contribution from the overall scattering pattern. The crystal thickness of PET was unperturbed upon blending with PEI. A larger amorphous layer thickness was identified in the blends, showing that some PEI was incorporated inside the interlamellar regions after crystallization. Despite the swelling of the amorphous layer, the amorphous layer thickness was relatively independent of the overall blend composition. Determination of the volume fraction of lamellar stacks from SAXS linear crystallinity and bulk crystallinity revealed the occurrence of significant extralamellar placement of PEI when the overall PEI composition exceeded 20 wt %. This morphological structure is interpreted in terms of the simultaneous occurrence of liquid-liquid demixing and crystallization.