High performance nanocomposites comprising a polyurethane elastomer (PUE) and an organically modified layered silicate are prepared. These nanocomposites are based on poly(propylene glycol), 4,4'-methylene bis(cyclohexyl isocyanate), 1,4-butandiol, and organoclay. The tensile strength and strain at break for these novel PUE nanocomposites increases more than 150%, but the hardness remains unchanged. The fatigue properties are significantly improved. With 3 wt % organoclay, the fatigue properties are improved the most, which is important for the PU industry. The effects of the isocyanate index on the mechanical properties of the PUE nanocomposites are investigated. It is found that an isoyanate index of 1.10 results in the best improvement in stress and elongation at break. (C) 2003 Wiley Periodicals, Inc.