Seashell waste, mainly composed of calcium carbonate and made of a complex microstructure, has been increasingly used as a bio–filler for composite materials, in a crushed or powdered state. Seashell particles present various sizes and shapes which in turn could affect the mechanical properties of the composites. The main aim of this work is to investigate the effect of particles size and shape and inter–phase properties on the overall elastic properties of bio–composite reinforced by seashell particles. For that purpose, a morphological study based on particles size measurement and shape analysis, was first carried out in order to identify the morphological distribution of seashell particles. Seashell particles were classified into six diameters and six types of shape. Afterwards, a numerical homogenization based on finite element and representative elementary volume is used to compute the effective elastic properties of bio–composites, which include bulk modulus, shear modulus, and Young’s modulus. We assumed a perfect bond at the polyester/seashell particle inter–phase. Then we incorporate an inter–phase surrounding the seashell particles by considering variable properties (thickness, number of layers). In the present work, the calculations are carried out for two types of microstructures according to particles shapes. Several microstructure configurations were considered by varying particles sizes and volume fractions 6.6, 13.7, 21.4, 25.5, and 29.8%. The results showed that adding seashell particles significantly improves the effective elastic properties of polyester composite and that the morphology of seashell particles and the inter–phase structure has a negligible effect on the effective elastic properties of bio–composite. As for inter–phase thickness, it could have a considerable effect on the effective elastic properties of bio–composites.