The effects of dioctyl phthalate (DOP) loading on morphology and thermal and mechanical properties of the poly(vinyl chloride)/organically modified montmorillonite/DOP (PVC/O-MMT/DOP) nanocomposites are studied in this paper. Results of transmission electron microscope (TEM) indicate that O-MMT is partially intercalated and exfoliated in all PVC/O-MMT/DOP nanocomposites. It shows that the addition of DOP has no obvious influence on the dispersion of O-MMT in the PVC/O-MMT/DOP nanocomposites. Young's modulus and tensile strength of the PVC/O-MMT/DOP nanocomposites are significantly decreased as the amount of DOP is increased. The thermal degradation onset temperature and glass transition temperature (T-g) of the PVC/O-MMT/DOP nanocomposites are decreased as the amount of DOP is increased. On the other hand, the elongation at break of the PVC/O-MMT/DOP nanocomposites is increased as the amount of DOP is increased. Also, the first thermal degradation weight loss (Delta Y) of the PVC/O-MMT/DOP nanocomposites is increased as the amount of DOP is increased. While increasing DOP loading, the PVC/O-MMT/DOP nanocomposites change from a rigid product to a semirigid product and finally into a flexible product.