The structural and morphological characteristics of biaxially oriented polyamide 12 films are described on the basis of the results from differential scanning calorimetry, wide-angle X-ray diffraction (WAXD), polarized FT-IR spectroscopy, and small angle X-ray scattering (SAXS). The WAY-D patterns of the oriented polyamide 12 films indicated only the monoclinic gamma crystal with little dimensional changes of its unit cell depending on the stretching conditions. The crystallographic angles (alpha = gamma = 90degrees, beta = 121degrees) that were determined via the WAXD patterns confirmed the monoclinic symmetry of the 7 crystal. Annealing the films stretched at 115 degreesC in boiling 20% formic acid solution did not result in structural changes of the crystalline unit cell. The chain-axis repeat distance of 31.9 Angstrom for the gamma crystal was experimentally obtained with (0 26 0) planes. It was shortened as compared with that of all-trans conformation. For films having primary orientation to MD, normals to the basal plane of folded-chain lamellae were parallel to MD (primary stretch direction) resulting in two-point SAXS patterns. Growth in long spacing with an increase of stretch temperature was discovered, Annealing the films induced further elongation in long spacing.