Unidirectional flax/polyethylene terephthalate composites are manufactured by filament winding, followed by compression moulding with low and high consolidation pressure, and with variable flax fibre content. The experimental data of volumetric composition and tensile stiffness are analysed with analytical models, and the composite microstructure is assessed by microscopy. The higher consolidation pressure (4.10 vs. 1.67 MPa) leads to composites with a higher maximum attainable fibre volume fraction (0.597 vs. 0.530), which is shown to be well correlated with the compaction behaviour of flax yarn assemblies. A characteristic microstructural feature is observed near the transition stage, the so-called local structural porosity, which is caused by the locally fully compacted fibres. At the transition fibre weight fraction, which determines the best possible combination of high fibre volume fraction and low porosity, the high pressure composites show a higher maximum performance in terms of tensile stiffness (40 vs. 35 GPa). The good agreement with the model calculations (fibre compaction behaviour, and composite volumetric composition and mechanical properties), allows the making of a property diagram showing stiffness of unidirectional flax fibre composites as a function of fibre weight fraction for consolidation pressures in the range 0–10 MPa.