Calcium carbonate was extracted from eggshell (ES) and its particle size was reduced in a three steps process resulting in three different sizes. Particles obtained were mixed with polypropylene in a Brabender mixer. The effect of particle size on thermal and mechanical properties and the use of stearic acid (SA) as a coupling agent were studied. Differential scanning calorimetry (DSC), thermogravimetrical analisys (TGA) and dynamic mechanic analysis (DMA) were employed in this research work. According to DSC results particle size and SA has a synergetic effect on crystallinity; the smaller the particle size, the higher the crystallinity, and the presence of SA also increased this property. It was needed a 20 %w of eggshell particles and SA to increased 5 °C the thermal stability of PP, for smaller ratios this property do not have significant changes. By the other hand, the coupling agent and just 0.5 %w of ES from the second attrition increased 800 MPa storage modulus of PP.