We investigate the effects of a nematic liquid crystalline polymer, Vectra A, on the structure and properties of its blends with a semicrystalline polymer, poly(ethylene terephthalate),PET. PET/Vectra blend composition ranged from 100/0 to 60/40. Real-time, in situ studies of isothermal and non-isothermal melt crystallizations of these blends were conducted using simultaneous wide and small angle X-ray scattering (WAXS and SAXS), differential scanning calorimetry (DSC), and quantitative polarizing optical microscopy. All techniques confirm that the addition of Vectra nematic liquid crystal delays the onset of crystallization,and affects the degree of crystallinity and structural parameters such as Bragg long period,lamellar thickness and linear stack crystallinity. SAXS results indicate that some of the Vectra component penetrates into the interlamellar regions of the crystal stacks. Vectra interrupts the entangled polymer network making it more difficult for lamellar crystals to nucleate. Slower nucleation and growth result in increased perfection of the PET crystals grown isothermally, but reduces the crystallization temperature of PET crystals grown non-isothermally causing these crystals to be less perfect. (C) 2005 Springer Science + Business Media, Inc.