The aim of the present study is to improve the understanding of the evolution of flax fibre (Linum usitatissimum L.) ultrastructure and properties caused by the retting process at the cell wall scale. The dew retting is a natural bioprocess, where the various microorganisms present in the soil colonize the flax stems. The enzymes they secrete make it easier to extract the bast fibre bundles during the scutching stage. In this study, six different stages of retting were investigated. Differences between retting stage 1 (R1, earlier) and the retting stage 6 (R6, over retted) are +17% and +23% with AFM PF-QNM and nanoindentation measurements, respectively. 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). Two complementary structural investigating technic will be presented, 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. One highlight of the work is that NMR results show a significant evolution in the cell wall ultrastructure of flax during the retting stages. The improvement observed in the mechanical performance of flax cell walls can be explained by the compaction of inaccessible zones. Based on the NMR results, a schematic representation of the flax cellulose elementary microfibrils structure of R1 and R6 was hypothesised. In this flax cellulose evolution model during retting, we have considered in particular the lateral fibril dimension (LFD) and the the lateral fibril aggregate dimension (LFAD). As illustration, the LFD of samples R1 and R6 are 4.1 and 4.5 nm, respectively, while the LFAD slightly decreases during retting from 17.1 nm for R1 to 16.3 nm for R6.