In recent years, lignocellulosic biomass has been increasingly used in various applications, often replacing petro-sourced materials. While for many of these applications the plant materials require coarse milling, some new applications for green chemistry, bio-energy and bio-packaging necessitate comminution to obtain very finely calibrated particles (below 200 m in size). This milling step is not inconsequential for lignocellulosic materials and can influence the physical (size, shape) and chemical characteristics (cellulose crystallinity, composition) of the powder. However, these different effects are still poorly understood. In this work, we study and elucidate the impact of intense and ultra-fine milling on the physico-chemical properties of plant fibres. Flax was chosen for this study because of its well-described hierarchical structure and biochemical composition in literature, making it a model material. Our main results evidence a strong impact of 0 to 23hrs ball milling on flax fibre morphology, especially on fibre aspect ratio falling from 20 to 5 but also on cell wall ultrastructure and composition. Cellulose content and crystallinity significantly decrease with milling time, leading to higher water sorption and lower thermal stability.