Composites of polypropylene (PP) and organically modified montmorillonite (org-V-MTT) were prepared by melt mixing in an intensive mixer. Three grades of PP's having different melt viscosities were employed to investigate the dispersion characteristics of the composites with various org-MMT's. Depending on the matrix viscosity and nature of the interlayer in org-MMT, significant variations of the phase structure were found. Under the constant mixing condition and matrix viscosity, intercalation of PP chains into the interlayer of org-MMT was possible when initial interlayer distance and packing density were maintained in the optimum range; by which the loss in entropy associated with the confinement of polymer chains was compensated The state of org-MMT particle dispersion was improved by increasing the matrix viscosity only in the case that dispersed phase is suitable for intercalation process thermodynamically, otherwise little variation was occurred regardless of the matrix viscosity. Due to the lack of specific interaction between PP and org-MMT considered here, although the intercalation was possible for an appropriate org-MMT, the composites revealed unstable phase structure upon increasing the mixing time, which was characterized by agglomeration of the org-MMT domains.