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 X-ray diffraction (XRD), morphology, and mechanical characteristics of rigid poly (vinyl chloride) (PVC)/Na+-MMT and PVC/O-MMT nanocomposites, respectively. Results of XRD and transmission electron microscopy (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 nanocomposites were improved simultaneously with adding 1-3 wt % Na+-MMT or O-MMT with respect to that of pristine PVC. However, the addition of Na+-MMT or O-MMT should be kept as not more than 3 wt % to optimize the mechanical properties and the processing stability of the rigid PVC/MMT nanocomposites. SEM micrographs of the fractured surfaces of the rigid PVC/Na+-MMT and PVC/O-MMT nanocomposites both before and after tensile tests were also illustrated and compared. (c) 2006 Wiley Periodicals, Inc.