The structure-property relationship between a biaxially oriented film from poly(ethylene terephthalate) and its fracture behaviour measured using the Trouser Tear method, has been explored. X-ray diffraction (XRD) was used to characterise the orientation distribution of crystalline and non-crystalline material in the plane of the film and compared with the fracture energy, G(c) measured in four directions during tearing. The fracture energy averaged over the four directions ranged between 12 and 25 kJ m(-2), and was found to correlate closely to the draw ratio during manufacture and therefore the degree of molecular orientation. However the individual values of G(c) displayed a further level of complexity. The expected anisotropic character of the fracture energy was found to change systematically as a function of position across the original width of manufactured film. This feature compared well with the underlying, crystalline orientation distribution and provided strong evidence that under the mode III deformation of the tear test, the fracture mechanism involves the amorphous-crystallite surface boundary. Further support for this mechanism was provided by a simple model which, based on this assumption was shown to predict reliably, the anisotropic character of the film. (C) 2004 Kluwer Academic Publishers.