The cellulose microfibril realignment of unitary flax fibres with contrasted density of structural defects, which are also known as dislocations and defined as zones of microstructure heterogeneities, was investigated upon tensile testing by means of X-ray diffraction performed on SWING beamline at synchrotron SOLEIL. The in situ continuous tensile tests demonstrate a microfibril angle (MFA) decrease ranging from 3 to 24% depending on the fibre, with initial MFA measured between 4.7◦ and 7.4◦. The correlation between both the initial and final MFA values and the defect density is further assessed thanks to polarized light microscopy measurements prior to tensile testing. The influence of twisting and initial orientation of the fibres are also highlighted and discussed. Both the heterogeneity of the MFA values along flax fibres and the cellulose microfibril reorientation upon stretching are evidenced by stepwise tensile testing with an X-ray beam vertical size reaching 20 μm. Indeed, initial MFA values vary between 4.5 and 17◦ along the fibres observed. The results are implemented in a finite element model in the elastic domain based on precise fibre morphologies obtained by X-ray microtomography. The numerical results quantify the influence of the microfibril realignment on the resulting apparent modulus, with a stiffening between 1.5 and 7.5% only partly explaining the non-linearities observed experimentally.