To exploit the potential of flax fibres in reinforcing polymers, the performance of flax fibres must first be understood and then optimized. In this context, this paper aims to provide a visual and comprehensive description of the impact flax fibre micro-structural features, such as kink bands, porosity and cortical residues, have on damage evolution during tensile loading of Polylactic acid (PLA) matrix composites reinforced by flax fibres. In-situ synchrotron radiation computed tomography (SRCT) has been used for 3D visualisation of microstructural evolution at stress levels between 10% and 90% of the ultimate failure stress. First, the main defects of the overall microstructure are described, including a quantitative analysis of porosities. Then, novel visual insights, highlighting the main role of kink-bands in fibre failure and subsequent composite breakage, are described. Interestingly, it appears that for the flax/PLA composite studied, kink-band inter-distances are consistently smaller than critical fibre lengths, 2 explaining the likelihood of rupture in kink-band regions. These findings demonstrate that fibre extraction and subsequent textile reinforcement manufacturing are critical steps and should be optimised to increase the performances of natural fibre composites.