Poly(ethylene oxide) (PEO)/organo clay nanocomposites were prepared by melt-mixing using a laboratory kneader followed by compression molding. The nanocomposites were characterized by X-ray diffraction, atomic force microscopy, and scanning electron microscopy. Their crystallization behavior in hot stage was investigated by polarized optical microscopy (POM). A decrease in size and regularity were observed as a result of incorporation of clay into the PEO matrix. The dynamic viscoelastic behavior of PEO/organo clay nanocomposites was assessed using a strain-controlled parallel plate rheometer. The effects of clay concentration and the processing temperature on the rheological properties of the nanocomposites were extensively studied. A significant increase in the viscosity and storage modulus of the nanocomposites was found with the increasing clay content. The flow activation energy decreased with the incorporation of clay. The reinforcing effect of the organoclay was determined in dynamic mechanical analysis and tensile testing. POLYM. ENG. SCI., 49:379-390, 2009. (C) 2008 Society of Plastics Engineers