In this article, a Haake torque rheometer equipped with an internal mixer is used to study the influence of the amount of sodium montmorillonite (Na+-MMT) and organically modified MMT (O-MMT) on the characteristics of rigid poly (vinyl chloride) (PVC)/Na+-MMT and PVC/O-MMT nanocomposites, respectively. It is observed that the fusion time and temperature of the rigid PVC/Na+-MMT nanocomposites are decreased with increasing the amount of Na+-MMT. On the contrast, the fusion time and temperature of the rigid PVC/O-MMT nanocomposites are increased with increasing the amount of O-MMT. Results of X-ray diffraction (XRD) and transmission electron microscope (TEM) indicate that MMT is partially encapsulated and intercalated in the rigid PVC/Na+-MMT nanocomposites. However, results of XRD and TEM show MMT is partially intercalated and exfoliated in the rigid PVC/O-MMT nanocomposites. Tensile strength, yield strength, and elongation at break of the rigid PVC/MMT (including PVC/Na+-MMT and PVC/O-MMT) nanocomposites were improved simultaneously with adding 1-3 wt % Na+-MMT or O-MMT with respect to those of pristine PVC. (c) 2005 Wiley Periodicals, Inc.