The micro/macro structural evolution of polyamide 66 fibres during tensile loading and failure initiation have been studied by coupling multi-scale measurements such as wide angle X-ray diffraction (WAXD) with profile fitting, birefringence, differential scanning calorimetry (DSC), micro-Raman spectroscopy and mechanical testing at two strain rates. A large dependence of mechanical properties, including strain rate effects, on the behaviour of both oriented and random amorphous regions has been shown with important contributions from the isotropic amorphous domains. The results indicate the presence of compressive residual stresses beneath the skin, showing a skin/core sub-structure. Statistical fracture treatments have been applied using a time-dependent weakest link Weibull model, in order to give an evaluation of the dispersion of defects. Local damage was taken into account using a crack growth propagation law as a function of stress intensity factor near the defect. The paper shows how both experimental and theoretical fracture toughness results are in quite good agreement. (C) 2003 Kluwer Academic Publishers.