ABSTRACT The aim of the present study is to improve the understanding of the evolution of fibre properties and ultrastructure caused by the retting process at the plant cell wall scale. Two complementary structural investigating technic are presented here, namely using X-Ray Diffraction (XRD) and solid state Nuclear Magnetic Resonance (NMR). An estimation of the cellulose crystallinity index by XRD measurements, confirmed by NMR, shows an increase of 8% in crystallinity with retting mainly due to the disappearance of amorphous polymer. In addition, NMR investigations show a compaction of inaccessible cell wall polymers, combined with an increase in the relaxation times of the C4 carbon. This densification provides a structural explanation for the observed improvement in mechanical performance of the secondary wall of flax fibres during the field retting process. INTRODUCTION During retting, the various microorganisms present in the soil colonize the flax stems. The enzymes they secrete, and particularly polygalacturonases, gradually degrade the pectic compounds located mainly in the middle lamellae of the fibre bundles. This damage to the plant tissues structure makes it easier to extract the fibre bundles during the scutching stage. An over retting results in the loss of the cellulose that makes up the fibres (Placet et al. 2017), arguably due to secretion of cellulase. To obtain information on the supramolecular structure of cellulose fibrils in the plant cell walls, the model of Larsson and Wickholm (Larsson et al., 1997) could be used for deconvoluate the C4 region between 77 and 92 ppm. In addition, the mechanical properties of flax cell walls during could be investigated by AFM Peak Force Quantitative Nano-Mechanical property mapping (AFM PF-QNM) (Goudenhooft et al. 2018). Here we explore the evolution of the flax cell wall behaviour during a controlled dew retting phase. Structural and mechanical investigations were conducted at the cell wall scale using XRay Diffraction (XRD) and solid-state Nuclear Magnetic Resonance (NMR), while nanomechanical measurements were performed by nanoindentation and AFM PF-QNM). RESULTS AND CONCLUSIONS The macroscopic illustration of the flax lots is displayed in Fig 1. NMR results show a significant evolution in the cell wall ultrastructure of flax during the retting stages. The improvement in the mechanical performance of plant cell walls can be explained by the compaction of inaccessible zones (Fig 2).