The dependence of fracture toughness on fibre orientation, in short fibre reinforced polymers, was investigated using materials with different polymer matrix (polyamide 6.6, polyarylamide and polyoxymethylene), fibre sizing, fibre content, mean fibre length and fibre length distribution. To assess the dependence on fibre orientation, plates with unidirectionally oriented fibres were prepared and cut at various angles with respect to the direction of the aligned fibres. The fracture behaviour was investigated by single-edge notch three-point bending tests. In addition the stress-strain behaviour was examined by performing uniaxial tension and compression tests. Both the critical stress intensity factor K-C and the fracture energy G(C) measured at fracture initiation were found to present a bi-linear relationship to the factor characterizing fibre orientation, with different slopes over different ranges of the orientation factor. This suggested the occurrence of a transition between different failure mechanisms with varying fibre orientation, namely matrix fracture and fibre debonding at low values of the fibre orientation factor, fibre breakage and pull-out at high values of the fibre orientation factor. This interpretation is supported by the observation of the crack growth direction ( which varies with varying fibre orientation) and the analysis of the fracture surfaces. The slopes of the two linear branches of the toughness vs. fibre-orientation-factor plot and the "critical" fibre orientation angle depend on all internal variables investigated: constituent polymer matrix, degree of fibre-matrix adhesion, fibre content, mean fibre length and fibre length distribution. (C) 2004 Kluwer Academic Publishers.