Flax shive (FS) is the predominant by-product of flax fibre production, representing around 50% of the weight of the dry flax stem. Recent studies [1] have shown that FS initial granulometry affects the tensile properties of the reinforced composite. Therefore, our study aims to investigate the influence of FS granulometry on the microstructure and mechanical properties of FS/Polypropylene (PP) injected composites. Various FS granulometries (FS-0-200, FS-200-315, FS-315-500, FS-200-500, FS-500-800 and FS-800-1600 µm) were prepared and incorporated into PP composites with fibre weight fraction ranging from 10 to 40%, using extrusion and injection moulding. An optimal quantity of Maleic Anhydride Polypropylene (MAPP) was used as a compatibilizer in the composites. X-ray tomography was conducted on composites reinforced with 10% wt. of FS-200-500 (Figure 1) and FS-800-1600, providing insights into the impact of FS granulometry on FS orientation and its distribution in the composite microstructure. The dimensions and aspect ratio of the FS particles were measured using a 2D scanner after solvent extraction. The results reveal that the maximum aspect ratio of the FS after processing is reached by the medium-size particles FS-200-315, with deviations in particle size leading to a decrease in its aspect ratio. Additionally, it was observed that FS-200-500 tends to be more oriented toward the injection flow compared to FS-800-1600. Composites reinforced with 30%wt. of FS-200-315 demonstrate a significant enhancement, with Young's modulus increasing by 19% (from 2964 MPa to 3519 MPa) and ultimate tensile strength by 26% (from 32.1 MPa to 40.3 MPa), compared to those reinforced with 30%wt. of FS-800-1600. The elastic properties of the composite were modelled using the Mori-Tanaka model based on the investigated microstructural parameters, allowing us to back-calculate the FS Young’s modulus. The Mori-Tanaka model shows that there is a variation in Young’s Modulus among the different FS granulometry within a range between 14 and 29 GPa (Figure 2), which is in the same range as estimated using analytical modeling in the literature: 27.9 GPa [2] and 22.2 GPa [3].