This study aims to investigate the reinforcement potential of flax shives (FS) in thermoplastic composites. The influence of FS initial granulometry on the microstructure of the composites and their tensile properties is studied. Various FS granulometries were prepared. Polypropylene (PP) composites reinforced with 10% to 40%wt. of FS for each granulometry were manufactured using twin-screw extrusion and injection-moulding processes with Maleic Anhydride Polypropylene (MAPP) as a compatibilizer. Microstructural analysis was carried out using 2D scanner after fibre extraction and Xray tomography on composites. The 3D microstructural parameters were used to model the elastic properties of the composite using Mori-Tanaka model. Our results revealed that FS-200-315 achieved maximum aspect ratio after processing, with granulometry deviations reducing aspect ratio. Additionally, medium size particles tend to be more oriented toward the injection flow compared to larger particles. These findings are consistent with the tensile properties of the composites. PP composites reinforced with FS-200-315 exhibit a 20% and 25% increase in Young’s modulus and ultimate tensile strength, respectively, compared to those reinforced with FS-800-1600. The Mori- Tanaka model shows that there is a variation in the back-calculated FS Young’s modulus as a function of granulometry, within a range between 14 and 28 GPa.