Composites of poly(vinyl chloride) (PVC) filled with micron- and nanosized calcium carbonate (CaCO3) particles were prepared by solution blending. The influences of particle size and CaCO3 content on the microstructure and mechanical properties of the PVC composites were investigated by means of polarized optical microscopy and mechanical testing. The polarized optical microscope images revealed that nanosized CaCO3 particles were more agglomerated than micron-sized CaCO3 particles and the amount of agglomerates increased with increasing particle content. PVC/CaCO3-0.22 composites (PVC nanocomposite filled with 220-nm-particle-sized CaCO3) 5 phr CaCO3 content had the maximum tensile strength. The Young's modulus of all composites increased with increasing particle content. The energy at break of all composites showed a decreasing trend as a function of CaCO3 content and varied with particle size.