Lamellar α-form single crystals of poly(butylene terephthalate) (PBT) have been prepared by our confined thin film melt polymerization method at polymerization temperatures of 190° to 210°C. These lamellar PBT single crystals have a thickness of ca. 100 Å, with (010) twinned crystals frequently observed. Six different zonal electron diffraction patterns were obtained, from which the lattice parameters are determined asa= 4.94 Å,b= 5.98 Å,c= 11.56 Å, α= 99.8°, β = 116.5°, and γ = 111.15° with a volume of 261.86 Å3, corresponding to a calculated density of 1.397 g/cm3for this “perfect” PBT single crystal. Chain packing and molecular conformation in the triclinic unit cell (space groupP1) were simulated by the Cerius2modeling software.