Peak-Force Quantitative Nano-Mechanical property mapping (PF-QNM) was applied to explore the nano-mechanical properties of flax fibre cell walls in cross-section. After validation of the ability of PF-QNM to determine stiffness gradients in aramid fibres, measurements were performed on developing flax bast fibres. The presence of two layers with different indentation moduli implies their progressive development during thickening of the secondary cell wall. Finally, measurements were carried out on technical flax fibre cell wall; but, in this case, no significant stiffness gradient could be identified in the secondary S 2 layer.