Effect of unmodified and surface-modified micro-/nano-calcium carbonate particles on the mechanical properties, crystallization behavior, and thermal properties of polypropylene (PP)/microscale calcium carbonate (micro-CaCO₃ (mCC)) and PP/nanoscale calcium carbonate (nano-CaCO₃ (nCC)) composites has been comparatively investigated via melt mixing. Mechanical tests indicated that PP/nCC composite with the addition of 5–10 wt% nCC is higher than that of virgin PP and even higher than that of PP/mCC composite with the addition of 5–15 wt% mCC. In addition, incorporation of titanate-coupling agent (isopropyl tri-(dioctylpyrophosphato) titanate (JN114)) further increased the mechanical properties of the composites. This improvement in the mechanical properties was evidenced by scanning electron microscopy. The addition of small amounts of JN114 into PP/mCC and PP/nCC induces the great change in crystallization behavior of PP matrix. Improved distribution of CaCO₃, enhanced crystallization temperature, largely increased the degree of crystallinity, X_c (%), and the heat deformation temperature are achieved for PP/modified-nCC samples.