In recent years, lignocellulosic biomass has been increasingly used in various applications. While for many of them 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 mu m in size). This milling step is not inconsequential for lignocellulosic materials and can influence the physical and chemical characteristics of the powder. However, these different effects are still poorly understood. In this work, we study the impact of ultra-fine milling on the physico-chemical properties of plant fibres. Flax was chosen for this study because of its well-described structure and biochemical composition, making it a model material. Our main results evidence a strong impact of ball milling on flax fibre aspect ratio 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. (C) 2019 Elsevier B.V. All rights reserved.