This work aims to elaborate composites based on polystyrene (PS) reinforced with CaO/CaCO3 particles produced from snail shell powder (SSP) using the free air casting method. The developed composites (PS/SSP) with different percentages of SSP (1, 5, 7, and 10% (by wt)), were characterized in terms of thermal, flow, mechanical properties, and structural and morphological examinations using differential scanning calorimetry (DSC), viscometry, tensile and hardness tests, and Fourier transform infrared (FTIR) and optical microscopy, respectively. Addition of SSP particles increased the viscosity of the PS matrix by 8. 21%, and the mechanical tests indicated 70% and 40% rise in Young's modulus and hardness, respectively, for the PS/SSP composite containing 10% (by wt) SSP particles. Thermal analysis revealed 36 and 16% rise in the glass transition (Tg) and the degree of cristallinity (XC) of the composites compared to pure PS, respectively. The morphological examinations showed that a 10% (by wt) of SSP particles were well dispersed within the PS matrix confirming, thus, the results obtained in the previous analyses. CaO/CaCO3 particles produced from SSP by mechanochemical process and heated thermally improved the PS properties. Therefore, SSP can be considered as a cheap and renewable biosource of CaO/CaCO3 particles that can be used for PS and other polymer matrices.