Polyurethane (PU) nanocomposites filled with attapulgite (ATT) nanorods were synthesized and characterized with thermal analysis, dynamic mechanical analysis (DMA), and mechanical testing. The formulations were based on 4,4'-methylene bis(phenyl isocyanate) (MDI), polytetrahydrofuran, 1,4-butanediol, and inorganic ATT premodified with MDI. The original and premodified ATT (ATT-OH and ATT-MDI) nanorods were characterized with thermogravimetric analysis (TGA) and Fourier transform infrared (FTIR) spectroscopy. The analysis revealed that 17 wt % MDI was grafted/adsorbed onto the surface of ATT as a result of the modification. Pristine PU and ATT-MDI/PU nanocomposites were characterized with scanning electron microscopy, differential scanning calorimetry, and TGA. The mechanical tests and DMA showed an increase in the storage modulus and Young's modulus with increasing ATT-MDI content. The crystallinity of the hard and soft segments and thermal stability showed enhancements over those of the neat resin. (C) 2008 Wiley Periodicals, Inc.