Poly(methyl methacrylate) (PMMA) and polystyrene (PS)/clay nanocomposites were prepared via in-situ bulk polymerization. The compatibility of the initiator and monomer with the clay surface was found to profoundly affect the clay dispersion. By using a surfactant containing a polymerizable group to modify the clay surface, exfoliated PMMA and PS/clay nanocomposites were synthesized. The clay dispersion was quantified by both X-ray diffraction (XRD) and transmission electron microscopy (TEM). The dimension stability of nanocomposites was studied and related to both nanoscale (layer separation) and mesoscale (long-range distribution) clay dispersion.