Polyurethane (PU) /organically modified layered silicate (organoclay) nanocomposites were prepared by in situ polymerization. The viscoelasticity of PU-organoclay nanocomposites was investigated. The results suggest that the addition of organoclay resulted in the increase in the elasticity of PU and the decrease in damping property, and improvement of the thermal stability of PU significantly. However, it was also found that the addition of organoclay did not enhance the modulus of PU significantly. It was found that at high contents of soft segments, the modulus of PU decreased with the addition of organoclay. Hysteresis results indicated that energy dissipation increased with increasing organoclay. (c) 2006 Wiley Periodicals, Inc.