A novel polyether polyurethane/clay nanocomposite was synthesized using polytetramethyleneglycol (PTMEG), 4,4'-diphenylmethane diisocyanate (MDI), 1,6-hexamethylenediamine,and modified Na+-montmorillonite. Here, organic-modified montmorillonite (O-MMT) was formed by applying 1,6-hexamethylenediamine as a swelling agent to treat Na+-montmorillonite. The X-ray analysis showed that exfoliation occurred for the higher O-MMT content (40 wt %) in the polymer matrix. Mechanical analysis indicated that when the O-MMT was used as chain extender to replace a part of 1,2-diaminopropane to form PU/clay nanocomposites, the strength and strain-at-break of the polymer were enhanced with increased content of O-MMT in the matrix. When the O-MMT content attained about 5%, the tensile strength and elongation at break were over two times that of the pure PU. The thermal stability and the glass-transition of O-MMT/PU nanocomposites also increased with increasing the O-MMT content. (C) 2004 Wiley Periodicals, Inc.