A Haake torque rheometer equipped with an internal mixer has been used to study the influence of microscale calcium carbonate (micro-CaCO3) and nanoscale calcium carbonate (nano-CaCO3) on the fusion, thermal, and mechanical characteristics of rigid poly(vinyl chloride) (PVC)/micro-CaCO3 and PVC/nano-CaCO3 composites, respectively. The fusion characteristics discussed in this article include the fusion time, fusion temperature, fusion torque, and fusion percolation threshold (FPT). The fusion time, fusion temperature, and FPT of rigid PVC/calcium carbonate (CaCO3) composites increase with an increase in the addition of micro-CaCO3 or nano-CaCO3. In contrast, the fusion torque of rigid PVC/CaCO3 composites decreases with an increase in the addition of micro-CaCO3 or nano-CaCO3. The results of thermal analysis show that the first thermal degradation onset temperature (T-onset) of rigid PVC/micro-CaCO3 is 7.5 degrees C lower than that of PVC. Meanwhile, the glass-transition temperature (T-g) of rigid PVC/micro-CaCO3 is similar to that of PVC. However, T-onset and T-g of PVC/nano-CaCO3 Composites can be increased by up to 30 and 4.4%, respectively, via blending with 10 phr nano-CaCO3. Mechanical testing results for PVC/micro-CaCO3 composites with the addition of 5-15 phr micro-CaCO3 and PVC/nano-CaCO3 composites with the addition of 5-20 phr nano-CaCO3 are better than those of PVC. (c) 2005 Wiley Periodicals, Inc.