Whatever in pulp or biocomposite sectors, the elements called fines coming from plant fibres have generally a length lesser than 200 μm. Their mechanical impact has long been debated in short plant fibre thermoplastic composites. Are they solely a filling agent or on the contrary, have they a potential of reinforcement in such composites depending on their numbers, their length and aspect ratio? This work proposes an original experimental approach to explore the mechanical role of fine flax particles. Based on controlled milling of an initially homogeneous flax fibre batch that provides a population of fines (99% under 200 μm, average Lw of 147 μm), we devised a set of composites made of an increasing content of flax fines (0, 3.1, 5.6, 12.3, 20.4, 34.6 and 40.2%-vol) mixed with poly(propylene) (PP) and maleic anhydrid grafted PP (PP-PPgMA). Reference composites reinforced with chalk and also with cut flax fibre (Lw of 2000 μm) were also manufactured and studied. Results demonstrate that, despite a low aspect ratio (5 ± 0.2 for 20.4%-vol), fines can act more than as just a simple filler, a slight modulus reinforcement is depicted but only beyond a high threshold of about 20%-vol (+29% compared to raw PP for 20.4%-vol). In addition to PP, we then investigated the mechanical influence of the flax fines in two representative matrix thermoplastic families Poly(amide)-11 (PA11) and poly(butylene-succinate) (PBS). We highlighted an increase of the Young’s modulus of PA11 and PBS fines composites (+41% and +115, respectively for 20.4%-vol), whereas strengths were lower compared to the respective neat polymers, exhibiting the possible negative role of fines on composite mechanical properties.