This paper aims to identify the main parameters that improve the flexural properties of long glass fiber/polyamide 66 injection moulded parts. The mould geometry has been chosen so as to reproduce some geometrical accidents (e.g. sharp frontal and tangential steps) occurring on industrial moulds. A Taguchi design of experiments (DOE) has been used in order to quantify the effects of processing conditions on the flexural strength and modulus. Polymer melt temperature is the main parameter acting on the flexural properties, in both flow and transverse directions. The structure/ process/ flexural properties relationship has then been deduced from microstructure analysis (crystallinity, local residual fiber length and average orientation, interfacial quality). For optimized injection moulding conditions, leading to the highest flexural strength in the flow direction, a fiber content gradient has been noticed over the part length and width, which is strongly amplified by the presence of any sharp geometrical discontinuity. The results have also pointed out the limits of the plasticating unit of the injection, moulding machine. A similarity with the reinforcement mechanisms of short fiber reinforced thermoplastics has been highlighted. However, in the case of long fibers the fiber orientation mechanism is more complicated due to tangling effects. © 2005 Wiley Periodicals, Inc.