Ultrasonication was used to control the dispersibility of organophillic montmorillonite (OMMT) and the rheological properties of poly(ethylene oxide) (PEO)/OMMT nanocomposites, which were prepared via the solvent casting method using chloroform as a cosolvent. X-ray diffraction analysis showed that the application of ultrasound to OMMT dispersions prior to the mixing with polymer solution increases the layer spacing in clay particles. We systematically developed the ultrasonification preparation procedure by establishing the relationship between dispersion quality and rheological properties of polymer/clay nanocomposites. Dispersion quality was compared with both thermal properties obtained from TGA and rheological properties measured in steady shear and dynamic oscillatory tests. To analyze the non-Newtonian flow behavior, we analyzed shear viscosity data via the Carreau model. We found that the steady shear viscosity and power-law index increase with the dispersion quality for a fixed OMMT content. The microstructure of PEO/OMMT nanocomposite having enhanced dispersibility through sonication was also examined using transmission electron microscopy.